In a speech on Tuesday, he also said the alliance is monitoring Russia’s nuclear forces closely as the country was “losing on the battlefield” in Ukraine.
Meanwhile, Moscow has issued a fresh warning to the West over its involvement in the Ukraine war.
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Deputy foreign minister Sergei Ryabkov said Russia will take adequate countermeasures in response to the West’s “growing involvement”.
In the comments reported by the state-owned RIA news agency, he said: “We warn and hope that they realise the danger of uncontrolled escalation in Washington and other Western capitals.”
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Image: NATO Secretary General Jens Stoltenberg said the alliance is monitoring Russia’s nuclear forces closely
Russian bombs have rained down on Ukraine, killing at least 14 people on Monday.
Despite remarkable Ukrainian battlefield successes – both early on in the war with the defence of Kyiv and more recently with counterattacks in Kharkiv and Kherson regions – the war could continue for decades to come, one expert said.
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September: NATO ‘could sink whole Black Sea Fleet’
Sky News security and defence analyst Professor Michael Clarke said the war is likely to be a “generational struggle” and could be a “forever conflict” until “something changes in European security or Russia”.
Mr Clarke said the current crisis in Ukraine was the “second war” and the first war was witnessed in 2014 when Russia annexed Crimea.
He added: “My feeling is next year there will be a ceasefire in which the Ukrainians will be better placed and that ceasefire will be unstable and it will break down and there will be a third war and then a ceasefire and a fourth war.
“We’re dealing here with an existential struggle because the Russian establishment thinks that Ukraine has no right to exist and they won’t change their mind in the short-term.
“This is likely to be a generational struggle. Let’s say it’s going to last 30,40 or 50 years.”
At least 20 people have been killed and dozens more injured after an Israeli airstrike targeting a school in Gaza, health authorities have said.
Reuters news agency reported the number of dead, citing medics, with the school in the Daraj neighbourhood having been used to shelter displaced people who had fled previous bombardments.
Medical and civil defence sources on the ground confirmed women and children were among the casualties, with several charred bodies arriving at al Shifa and al Ahli hospitals.
The scene inside the school has been described as horrific, with more victims feared trapped under the rubble.
This breaking news story is being updated and more details will be published shortly.
Donald Trump has threatened Russia with more sanctions after a series of deadly strikes across Ukraine, as he said of Vladimir Putin: “What the hell happened to him?”
Speaking to reporters at an airport in New Jersey ahead of a flight back to Washington, Mr Trump said: “I’m not happy with Putin. I don’t know what’s wrong with him.”
“He’s killing a lot of people,” he added. “I’m not happy about that.”
Mr Trump – who said he’s “always gotten along with” Mr Putin – told reporters he would consider more sanctions against Moscow.
“He’s sending rockets into cities and killing people, and I don’t like it at all,” he said.
Ukraine said the barrage of strikes overnight into Sunday was the biggest aerial attack of the war so far, with 367 drones and missiles fired by Russian forces.
It came despite Mr Trump repeatedly talking up the chances of a peace agreement. He even spoke to Mr Putin on the phone for two hours last week.
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2:38
Hundreds of drones fired at Ukraine
‘Shameful’ attacks
Volodymyr Zelenskyy has said Ukraine is ready to sign a ceasefire deal, and suggested Russia isn’t serious about signing one.
In a statement after the latest attacks on his country, he urged the US and other national leaders to increase the pressure on Mr Putin, saying silence “only encourages” him.
Mr Trump’s envoy for the country, Keith Kellogg, later demanded a ceasefire, describing the Russian attacks as “shameful”.
Three children were among those killed in the attacks, explosions shaking the cities of Kyiv, Odesa, and Mykolaiv.
Image: Ukrainian siblings Tamara, 12, Stanislav, eight, and Roman, 17, were killed in Russian airstrikes. Pic: X/@Mariana_Betsa
Before the onslaught, Russia said it had faced a Ukrainian drone attack on Sunday. It said around 100 were intercepted and destroyed near Moscow and in central and southern regions.
The violence has escalated despite Russia and Ukraine completing the exchange of 1,000 prisoners each over the past three days.
Donald Trump says he will delay the imposition of 50% tariffs on goods entering the United States from the European Union until July, as the two sides attempt to negotiate a trade deal.
It comes after the president of the European Commission, Ursula von der Leyen, said in a post on social media site X that she had spoken to Mr Trump and expressed that they needed until 9 July to “reach a good deal”.
But Mr Trump has now said that date has been put back to 9 July to allow more time for negotiations with the 27-member bloc, with the phone call appearing to smooth over tensions for now at least.
Speaking on Sunday before boarding Air Force One for Washington DC, Mr Trump told reporters that he had spoken to Ms Von der Leyen and she “wants to get down to serious negotiations” and she vowed to “rapidly get together and see if we can work something out”.
The US president, in comments on his Truth Social platform, had reignited fears last Friday of a trade war between the two powers when he said talks were “going nowhere” and the bloc was “very difficult to deal with”.
Mr Trump told the media in Morristown, New Jersey, on Sunday that Ms Von der Leyen “just called me… and she asked for an extension in the June 1st date. And she said she wants to get down to serious negotiation”.
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“We had a very nice call and I agreed to move it. I believe July 9th would be the date. That was the date she requested. She said we will rapidly get together and see if we can work something out,” the US president added.
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0:54
12 May: US and China reach agreement on tariffs
Much of his most incendiary rhetoric on trade has been directed at Brussels, though, even going as far as to claim the EU was created to rip the US off.
Responding to his 50% tariff threat, EU trade chief Maros Sefcovic said: “EU-US trade is unmatched and must be guided by mutual respect, not threats.