Members of a Ukrainian brigade have described how they were secretly relocated to help defend a section of the country’s border with Russia a few days before a new invasion began.

The commander of an artillery unit from 57 Brigade said his guns were even firing at Russian troops the day before the ground incursion into the northeastern region of Kharkiv, which started on 10 May. He said the forces had been “brazenly” amassing on the Russian side of the border.

“We were hitting tanks on the border… it was already a real war,” said Sasha, 26, who uses the callsign “black”.

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The commander of a second artillery unit similarly confirmed the brigade had been moved early to bolster defences in this direction.

The troops had previously been defending the city of Kupiansk, also in Kharkiv.

The comments offer a sense of how Ukraine attempted – ahead of time – to scramble forces to counter a Russian build-up along its long, northeastern border.

But the move was nowhere near enough to prevent the largest assault into Ukraine since Russia launched its full-scale invasion almost two and a half years ago.

A Ukrainian source, describing the first few days of the Kharkiv offensive just over a week ago, said there had been moments when he feared “we had lost the frontline”.

The source said the situation had since stabilised but warned: “We don’t know how long it could be like this”.

Sky News on Saturday tried to visit an artillery position, manned by soldiers from 57 Brigade, just outside the town of Vovchansk – a key target of the Russian offensive.

As we approached at speed by road, a soldier travelling with us said we had to pull over because he needed to communicate via radio with troops on the gunline.

Suddenly a voice over the radio could be heard saying: “Don’t come here. Don’t come at all.”

We were told it was too dangerous to travel further and we had to leave. It was not immediately clear what was happening on the ground.

At a makeshift base, safely back from the frontline, the artillery unit commander Sasha uses electronic maps on a tablet and laptop to confirm targets for his guns to attack.

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He said he and his team relocated from the Kupiansk front on the 4 to 5 May.

“We were indeed moved here earlier,” Sasha said. But he signalled he would have liked longer to prepare.

“I don’t know all the situation and why it happened like this. But I know for sure that to better repel [an attack], we might need either more time or better-prepared positions,” he said.

“Ahead of the assault, we were already hitting targets on Russian territory because we knew they were gathering there. They were brazenly assembling.”

Sasha described the moment the Russians started to advance.

He said it began with three hours of artillery fire against Ukrainian targets before ground troops crossed the border.

“I would love that they [the Russians] had been stopped at the border,” he said.

Instead, a fierce battle erupted, as Russian infantry, backed by airstrikes, drone attacks and artillery fire, pushed forward.

“For the first few days, they [the Russians] were storming our positions – columns of 30 to 50 soldiers. We were hitting them.”

In the chaos, Sasha said he worked to gather information to ensure his troops were able to operate.

“I am proud that my guys managed to do their best,” he said. “All credit to those who stayed on their artillery positions.”

He described the frontline as initially being “fragile” but said reinforcements were now in place. The commander said Russia had lost the opportunity to make a significant breakthrough.

“Until now they had a chance. Even in my area, I knew where we had gaps where they could have slipped through. Now we don’t have such gaps,” he said.

“I am satisfied that we have managed to stabilise the situation.”

At a second artillery position, on a different section of a frontline that has expanded by some 40 miles in the wake of the new attack, a Soviet-era gun, hidden under netting and tree branches, points in the direction of Russia.

Soldiers here said they would be able to inflict a lot more damage on the invaders if they had more ammunition and better weapons.

Nicknamed “grandma”, their D-20 Howitzer artillery piece, which fires 152mm shells, was built in the 1970s.

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“We’re saving our artillery shells right now. We fire one, they fire back five,” said one of the servicemen, who – at 50 years old – has the callsign “Grandpa”.

A second soldier said Russia has more weapons than his side.

Asked what difference additional munitions would make, he said with a laugh: “It would increase the number of dead Russians – 100%”.

Additional reporting by Azad Safarov, Ukraine producer

While fighting continues in the east, a new front has been opened in the north of Ukraine.

In the early hours of Friday 10 May, Russian ground troops, backed by warplanes, drones and artillery, crossed the Ukrainian border into Kharkiv region.

In recent months, fighting in Ukraine has been focused across front lines in the east of the country. This development has focused attention on Ukraine’s northeast and its second city of Kharkiv, which lies less than 20 miles from the border.

Fighting around Vovchansk

Footage captured in the early stages of the Russian offensive shows a group of Ukrainian soldiers fighting in woodland.

Sky News has geolocated a section of this clip, which shows the troops withdrawing south, away from the border.

Other video provided to Sky News by Ukrainian sources sheds further light on how fighting unfolded in the early stages of Russia’s assault from the north.

Filmed from a drone, footage shows a group of Russian soldiers on Tuesday entering Vovchansk along the main road connecting to the town from the northwest.

In separate footage, a group can be seen moving southwest around 140 metres from this position. A further 400 metres in this direction, a group was filmed being targeted by Ukrainian forces while appearing to carry one of their wounded.

Other videos shared by the Ukrainians show that by this time, some Russian forces had moved from the outskirts into a residential area in the west of Vovchansk. They can be seen moving between buildings and are filmed being targeted by a Ukrainian drone.

Footage posted on 16 May suggests Russian troops in the area have been able to make steady progress through the town from the northwest. A group was filmed outside the town’s hospital, with one falling from the building’s first floor.

While on the ground, fighting in Vovchansk appears to have been relatively contained to the west of the town, it has been subject to intense bombing in recent days.

Drone footage posted on 14 May captured smoke rising from multiple locations across the town, showing the scale of Russian shelling.

Just west of Vovchansk, Russian forces entered the villages Buhruvatka and Ohirtseve.

In a photograph geolocated to Ohirtseve by Sky News, a group of soldiers posed with a flag bearing the face of Chechnya’s former leader Akhmad Kadyrov, who was killed in 2004.

Russians advance towards Lyptsi

Further west, Russian forces have made progress in the direction of the town of Lyptsi. Located just south of the Russian border, the town is a mere 20km from the city of Kharkiv.

Geolocated images suggest some Russian forces approached the Ukrainian border from multiple locations. One posted on 10 May shows a collection of destroyed Russian vehicles just north of the Ukrainian border town Pylna.

Footage released on 11 May suggests this was not the only point at which Russians attempted to cross the border, as the video shows a Ukrainian drone targeting Russian vehicles between the towns of Strilecha and Krasne.

From the north, Russian troops moved towards Lyptsi along a road running southwest from the border. Videos posted in recent days show they were targeted repeatedly by Ukrainian drones along this route.

Drone footage posted on 15 May shows the scale of shelling that accompanied this ground offensive, targeting Ukrainian positions ahead of Russian forces.

While Russia has made significant progress in their push south, Ukrainian forces have fought back. Footage posted on 15 May shows a Russian position just north of Lyptsi being bombed and approached by a group of Ukrainian soldiers, who are filmed capturing surviving Russians.

A ‘buffer zone’

While it is yet to be seen how far Russian troops will advance from the north, Vladimir Putin has claimed that the offensive is not an effort to capture the city of Kharkiv.

In a statement on 17 May, he said: “I have said publicly that if it continues, we will be forced to create a security zone, a sanitary zone,” claiming this was the current goal of Russian troops in the north of Ukraine.

For the moment, most fighting is still concentrated in the east, where Russia occupies vast areas of Ukrainian territory.

Oleksandr Usyk has become the undisputed heavyweight champion of the world after defeating British boxing star Tyson Fury.

The Ukrainian won on a split decision following the match in Riyadh, Saudi Arabia.

Usyk had 115-112 and 114-113 on two cards, while Fury took the other 114-113.

Follow the match as it happened

Fury disputed his loss after the match, saying: “I believe I won that fight. I think he won a few rounds but I won the majority of them.

“His country is at war, so people are siding with the country at war. Make no mistake, I won that fight in my opinion.

In response Usyk said he was “ready for rematch,” but later added: “I don’t think about rematch now, I want to rest.”

Fury came under early pressure, with Usyk taking the centre of the ring with an aggressive offensive from the start.

At one point Fury was pushed against the ropes and started laughing as Usyk applied pressure.

The “Gypsy King” looked relaxed as he moved around the ring in the early rounds and picked his shots.

But after Usyk landed a right hook in the ninth round it looked as if Fury was in serious trouble.

The Ukrainian followed up by unloading freely but somehow the bookmakers’ favourite stayed on his feet and was given a standing 10-second count saved by the bell.

It left Fury struggling through the final three rounds as Usyk chased him around the ring.

The 37-year-old Ukrainian became the first boxer to hold all four major heavyweight belts at the same time and the first undisputed champion in 24 years.

He’s the best fighter of a generation, there’s nothing left

Jacquie Beltrao

Sports presenter

There’s something very special about Oleksandr Usyk and it’s something all brilliant sports people have: the ability to find that extra bit of grit, to dig a bit deeper, when the battle is slipping away.

It’s exactly the character he showed, coming back at Fury in the 7th and 8th rounds, with some impressive shots, to take the sting out of any Fury resurgence and to swing momentum back his way. And enabling him to go for the kill in that brilliant 9th round. Fury looked stung, he looked confused and he was lucky the referee didn’t stop the fight there and then.

It was amazing that Fury made it to the end. That took courage. But it’s hard to see how he’s going to recover from this. It’s going to hurt. He says he wants to invoke the rematch clause and go again, but will he really want to?

Will Usyk want to? He’s the best fighter of a generation, there’s nothing left to prove. No fighter has ever won the undisputed cruiserweight championship of the world and followed that with the undisputed heavyweight crown. He can take four belts back to Kyiv safe in the knowledge that it’s unlikely anyone will be able to match that achievement anytime soon.

Last night, Fury weighed in at 262lbs (18st 10lbs) – nearly three stone heavier than Usyk, who clocked in at a career heaviest of 223lbs (15st 13lbs).

Fury refused to look at his opponent during a news conference on Thursday, but did not back down at the weigh-in last night, where the pair almost came to blows before being separated by their entourages.

Usyk arrived into the ring first, dressed as a Cossack warrior.

Fury entered to songs by Barry White and Bonnie Tyler, with the “Gypsy King” spending several minutes dancing on stage before the song changed to Holding Out For A Hero.

Anthony Joshua watched from the ringside, knowing he could meet the winner early next year.