European leaders including Sir Keir Starmer have threatened Vladimir Putin with fresh sanctions if Russia fails to comply with an unconditional 30-day ceasefire.
The prime minister met Ukrainian President Volodymyr Zelenskyy alongside French President Emmanuel Macron, recently-elected German Chancellor Friedrich Merz and Polish Prime Minister Donald Tusk in Kyiv on Saturday.
It is the first time the leaders of the four countries have travelled to Ukraine - arriving by train - at the same time.
They updated Donald Trump on the progress made on the so-called "coalition of the willing" plans in a 20-minute phone call.
Following the summit, the leaders announced an agreement that there should be an unconditional 30-day ceasefire starting on Monday, with the backing of the US president.
"All of us here, together with US, are calling Putin out," said Sir Keir.
"So we are clear, all five leaders here - all the leaders of the meeting we just had with the coalition of the willing - an unconditional ceasefire, rejecting Putin's conditions, and clear that if he turns his back on peace, we will respond.
"Working with President Trump, with all our partners, we will ramp up sanctions and increase our military aid for Ukraine's defence to pressure Russia back to the table."
Russia is reportedly considering the ceasefire proposal.
Kremlin spokesman Dmitry Peskov told CNN on Saturday: "We have to think about that. These are new developments. We have our own position."
It comes after Donald Trump called for "ideally" a 30-day ceasefire between Kyiv and Moscow, and warned that if any pause in the fighting is not respected "the US and its partners will impose further sanctions".
Security and defence analyst Michael Clarke told Sky News presenter Samantha Washington the European leaders are "rowing in behind" the US president, who referred to his "European allies" for the first time in this context in a post on his Truth Social platform.
"So this meeting is all about heaping pressure on the Russians to go along with the American proposal," he said.
"It's the closest the Europeans and the US have been for about three months on this issue."
Mr Zelenskyy told reporters the agreed ceasefire should cover air, sea and land, and said that if Moscow refused, Russia would face new sanctions, including the strengthening of punitive measures targeting its energy and banking sectors.
The European leaders said the terms of a peace deal would be negotiated during the 30-day pause in fighting.
But the Ukrainian president said: "We have no illusions that the ceasefire will be breached."
Mr Macron said the proposed ceasefire would be monitored mainly by the US and European countries and there would be "massive" sanctions if Russia did not agree.
Military officers from around 30 countries have been involved in drawing up plans for a coalition, which would provide a peacekeeping force in the event of a ceasefire being agreed between Russia and Ukraine.
This force "would help regenerate Ukraine's armed forces after any peace deal and strengthen confidence in any future peace", according to Number 10 Downing Street.
Mr Peskov accused European leaders of making contradictory and confrontational statements, according to Interfax news agency.
"We hear many contradictory statements from Europe. They are generally confrontational in nature rather than aimed at trying to revive our relations. Nothing more," he was quoted as saying.
As the European leaders pulled into Kyiv by train on Saturday, the screen on the platform announced the arrival of the "Bravery Express".
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Mr Zelenskyy accompanied them as they paid their respects at a memorial in central Kyiv to honour Ukrainian soldiers killed in the current war.
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The visit came on the final day of a three-day ceasefire unilaterally declared by Mr Putin, which was denounced as a sham by Ukraine.
Both sides have accused each other of violating it.
(c) Sky News 2025: Ukraine and 'coalition of the willing' press Russia for 30-day ceasefire from Monday