The Kremlin has reacted icily to Donald Trump's warnings to President Vladimir Putin over Ukraine, saying that recent decisions by the US president and the NATO military alliance would be interpreted by Kyiv as a signal to continue the war.

Trump, sitting beside NATO Secretary General Mark Rutte in the Oval Office, on Monday, announced new weapons for Ukraine and threatened "biting" secondary tariffs of 100 per cent on the buyers of Russian exports unless there is a peace deal in 50 days.

The US president also expressed frustration with Vladimir Putin, saying he did not want to call the Russian leader "an assassin, but he's a tough guy".

Trump said he is disappointed but not done with Putin, according to a BBC interview, which was published hours after Trump announced new weapons for Ukraine and threatened Russia with sanctions.

Trump told the BBC he thought a Ukraine deal was on the cards with Russia four different times.

Asked then by the BBC was he done with Putin, Trump said: "I'm disappointed in him, but I'm not done with him."

"We'll have a great conversation. I'll say: 'That's good, I'll think we're close to getting it done', and then he'll knock down a building in Kyiv." 

Kremlin spokesman Dmitry Peskov told reporters: "We certainly need time to analyse what was said in Washington".

"The US president's statements are very serious. Some of them are addressed personally to President Putin."

Peskov, though, added that it was already clear that decisions being made in Washington and other NATO capitals were "perceived by the Ukrainian side not as a signal for peace but as a signal to continue the war."

Trump expresses growing exasperation with Putin over Ukraine. – AP

"Trump issued a theatrical ultimatum to the Kremlin," former Russian President Dmitry Medvedev said in a post in English on X.

"The world shuddered, expecting the consequences. Belligerent Europe was disappointed. Russia didn’t care."

Trump, who has said he wants to be seen as a "peacemaker" president, said he wanted to see the end of the war - on which he said the United States had spent $350 billion.

Putin, who has spoken to Trump by telephone at least six times this year, has yet to comment publicly on Trump's remarks.

During his meeting with Rutte in the Oval Office, Trump told reporters he was disappointed in Putin and that billions of dollars of US weapons would go to Ukraine.

"We're going to make top-of-the-line weapons, and they'll be sent to NATO," Trump said, adding that Washington's NATO allies would pay for them.

The weapons would include Patriot air defence missiles Ukraine has urgently sought.

"It's a full complement with the batteries," Trump said. "We're going to have some come very soon, within days."

"A couple of the countries that have Patriots are going to swap over and will replace the Patriots with the ones they have."

"We have one country that has 17 Patriots getting ready to be shipped. We're going to work a deal where the 17 will go or a big portion of the 17 will go to the war site."

Doubts were recently raised about Trump’s commitment to supply Ukraine when the Pentagon paused shipments over concerns that US stockpiles were running low.

Rutte said Germany, Finland, Canada, Norway, Sweden, the United Kingdom and Denmark would be among the buyers to supply Ukraine. He said “speed is of the essence here”, and he suggested that some weapons would be rushed to Ukraine and later replaced with purchases from the US

Trump and Rutte announce 'really big' deal for US to send Ukraine weapons through NATO. – AP

Later Monday, Zelensky posted about having spoken with Trump by phone and said he “discussed the necessary means and solutions with the President to provide better protection for people from Russian attacks and to strengthen our positions”.

Zelensky added that Trump had “agreed to catch up more often by phone and coordinate our steps in the future”.

Zelensky said he had thanked Trump for his support and said he also had a very good conversation with Rutte..

"It was a very good conversation. I thanked him for his readiness to support Ukraine and to continue working together to stop the killings and establish a lasting and just peace," Zelensky wrote online.

Zelensky had earlier met US special envoy to Ukraine Keith Kellogg in the Ukrainian capital Kyiv, where they discussed boosting Ukraine's air defences and Kyiv buying weapons with European help.

Trump had said on Sunday that he would send Patriot air defence missiles to Ukraine.

"We discussed the path to peace and what we can practically do together to bring it closer. This includes strengthening Ukraine's air defence, joint production, and procurement of defence weapons in collaboration with Europe," Zelensky wrote online.

"And of course, sanctions against Russia and those who help it."

Trump, who began his second term with a more conciliatory approach to Russia, has in recent weeks signalled disenchantment with Putin as Moscow has stepped up air strikes on Kyiv and other Ukrainian cities.

An air-raid alert was declared in Kyiv shortly after the meeting between Kellogg and Zelensky on Monday.

Rutte said Germany, Finland, Denmark, Sweden, Norway, the United Kingdom, the Netherlands and Canada all wanted to be a part of rearming Ukraine.

Trump's threat to impose so-called secondary sanctions on Russia, if carried out, would be a major shift in Western sanctions policy. Lawmakers from both US political parties are pushing for a bill that would authorise such measures, targeting other countries that buy Russian oil.

Trump's threat of sanctions came with a 50-day grace period, a move that was welcomed by investors in Russia where the rouble recovered from earlier losses and stock markets rose.

Throughout the more than three-year-old war, Western countries have cut most of their own financial ties to Moscow, but have held back from taking steps that would restrict Russia from selling its oil elsewhere. That has allowed Moscow to continue earning hundreds of billions of dollars from shipping oil to buyers such as China and India.

"We're going to be doing secondary tariffs," Trump said. "If we don't have a deal in 50 days, it's very simple, and they'll be at 100 per cent."

A White House official said Trump was referring to 100 per cent tariffs on Russian goods as well as secondary sanctions on other countries that buy its exports. Eighty-five of the 100 US senators are co-sponsoring a bill that would give Trump the authority to impose 500 per cent tariffs on any country that helps Russia, but the chamber's Republican leaders have been waiting for Trump to give them the go-ahead for a vote.

In Kyiv, people welcomed the announcement but some remained cautious about Trump’s intentions.

“I am pleased that finally European politicians, with their patience and convictions, have slightly swayed him (Trump) to our side, because from the very beginning it was clear that he did not really want to help us,” said Denys Podilchuk, a 39-year-old dentist in Kyiv.

AP EXPLAINS: Trump's transition to supporting Ukraine. – AP

Asked about Trump’s remarks, UN Secretary-General Antonio Guterres said an immediate ceasefire was needed to pave the way for a political solution and "whatever can contribute to these objectives will, of course, be important if it is done in line with international law".

Since returning to the White House promising a quick end to the war, Trump has sought rapprochement with Moscow, speaking several times with Putin. His administration has pulled back from pro-Ukrainian policies such as backing Kyiv's membership in NATO and demanding Russia withdraw from all Ukrainian territory.

But Putin has yet to accept a proposal from Trump for an unconditional ceasefire, which was quickly endorsed by Kyiv. Recent days have seen Russia use hundreds of drones to attack Ukrainian cities.

Trump said his shift was motivated by frustration with Putin, who talked about peace but continued to strike Ukrainian cities.

"I don't want to say he's an assassin, but he's a tough guy," he said.

"We actually had probably four times a deal. And then the deal wouldn't happen because bombs would be thrown out that night and you'd say we're not making any deals," he said.

US Senator Jeanne Shaheen, the top Democrat on the Senate Foreign Relations Committee, said Trump's announcement was "positive, but overdue" and he needed to commit "to a sustained flow of security assistance to Ukraine over the long term" if he wanted Putin to negotiate and the war to end.

Volodymyr Zelensky and Donald Trump. – file

Artyom Nikolayev, an analyst from financial information firm Invest Era, said Trump did not go as far as Russian markets had feared.

"Trump performed below market expectations. He gave 50 days during which the Russian leadership can come up with something and extend the negotiation track. Moreover, Trump likes to postpone and extend such deadlines," he said.

Evelyn Farkas, a former senior Pentagon official who is now executive director of the McCain Institute, said Trump's moves could eventually turn the tide of the war if Trump ratchets up enforcement of current sanctions, adds new ones and provides new equipment quickly.

"If Putin's ministers and generals can be convinced that the war is not winnable they may be willing to push Putin to negotiate, if nothing else but to buy time," she said.

Russia began its full-scale invasion in February 2022 and holds about one-fifth of Ukraine. Its forces are slowly advancing in eastern Ukraine and Moscow shows no sign of abandoning its main war goals.