In a bunker below the fiercely contested city of Bakhmut, a jihadi commander with a half a million-dollar Russian bounty on his head, joins his men in prayer.

Of the allies Ukraine has gathered in its war with Russia, among the most shadowy and deadly are the Chechens.

They are some of Vladimir Putin’s oldest enemies and among the hardest to film up close.

Read more: Chilling moment Russian soldiers scour Ukrainian orphanage for children

They are all marked men, wanted by Russia. Their movements are shrouded in secrecy. But Sky News gained access to their secret base near the frontline in one of Ukraine’s most savage battles.

During the time we spent filming them they shared insights into their foes that are worth listening to in the West.

We drove in fast on back roads to evade Russian spotters calling in artillery strikes. As we entered Bakhmut we passed gutted buildings and gaping craters, the sound of shelling was close and regular.

Inside the bunker we met some of the longest serving veterans of this war. The Chechen Sheikh Mansour Battalion has been fighting Russia in Ukraine since 2014. Their enemy’s tactics haven’t changed since this war began, they say.

“They’re sending forward troops like cattle for slaughter,” Chechen fighter Idris told us. “Leaving the ground covered with corpses. They do it every day they have no pity for their own people.”

It is the same kind of fighting Russia used in their homeland in the 1990s. From safe cover, commanders send conscripts in waves hoping to grind down their enemy with little care for their men.

Chechens have been fighting for an independent country since the collapse of the Soviet Union.

After victory in the first Chechen war they were defeated by Russia, and Vladimir Putin installed a puppet leader, Ramzan Kadyrov, propping him up with billions of dollars in support.

He combines brutal repression with self-promotion on social media that veers from the sinister to the preposterous.

His Chechen forces fight on the side of Russia in this war. His Chechen enemies on the other. The conflict has given Chechen separatists a new arena for their struggle against their enemy.

Commander Muslim Cheberloyevskii leads the Sheikh Mansour battalion, one of their most active fighting units. He gives very few interviews but made an exception for Sky News. Before joining his men in Bakhmut we spoke to him on a video link to his secret location elsewhere in Ukraine.

“There can be no options here,” he said. “Russia must lose, and it must end there. If we do not defend Ukraine today, everyone will lose.”

The man who has fought Putin’s forces longer possibly than any other commander, used the interview to warn policy makers in the West they are not doing enough to defeat him.

“I think more support is necessary. The West provides support by portions, they are limited. Munitions are quickly used, they are not enough on the battlefield. If we had more, we could win quicker.”

In the bunker under Bakhmut there was the same message. Base commander Mansour has spent two decades fighting the Russians, eight of them spent in jail where he was tortured.

“I have no pity for them at all,” he said of his enemy. “Because God gave everyone a brain for thinking. If he’s not thinking he shouldn’t walk on the earth, he belongs below the ground.”

And he warned Western leaders not to fall for Putin’s enticements to negotiate an end to this war.

“Even when they agree to negotiate and sign some documents, they do not follow them, they act treacherously.”

They are a sabotage unit, using weapons, some improvised, to strike the enemy in their trenches. Commander Mansour showed us a homemade rocket propelled grenade fashioned from a fire extinguisher packed with plastic explosives.

On a work bench nearby a suicide vest was being constructed. They wear them should Russians take them prisoner. The base is mined, they said, to blow up if the enemy should overrun it.

In an outbuilding, Deputy Commander Mansour showed off what he called his “Devil’s machine”, a rocket launcher improvised to fire converted mine clearance shells.

On his phone he shared video of the device in action at night. A fiery launch followed by a pause then a huge explosion in the distance lighting up the sky with a mushroom cloud of fire. The fighters shout Allahu Akbar: God is great.

They fight here hoping one day to take their holy war back to their homeland. Kadyrov is unpopular but well-funded and protected by thousands of well-armed security forces. When the war is over though they say they will continue fighting Russia, hoping to topple him.

Asadullah, a Ukrainian who converted to Islam and joined the battalion speaks for many of them.

“If today the war ends in Ukraine, and we win, for us it will not end,” he said.

“We will fight till that time when we destroy that empire of evil totally.”

For now though, that is a very long way off. We left their bunker and drove again at speed out of Bakhmut to a backdrop of artillery fire. Their enemy is destroying another Ukrainian city block by block in a grinding war of attrition no one looks close to winning.