Prime Minister Boris Johnson has travelled to Kyiv, to have talks with Ukrainian President Volodymyr Zelensky.
During the visit, Mr Johnson offered a UK-led military training programme which he said could “change the equation of the war”.
No 10 said it would help Ukraine’s “valiant fight” against “the vain ambitions of President Putin”.
Mr Zelensky welcomed Mr Johnson to Kyiv describing the PM as his country’s “great friend”.
This is the prime minister’s second visit to Ukraine since the Russian invasion in February.
Mr Johnson announced his visit by tweeting a picture of himself with the Ukrainian president with the message: “Mr President, Volodymyr. It is good to be in Kyiv again.”
Mr Zelensky’s Telegram account posted a message saying: “Many days of this war have proved that Great Britain’s support for Ukraine is firm and resolute.
“Glad to see our country’s great friend Boris Johnson in Kyiv again.”
Downing Street said the UK was offering a programme to train up to 10,000 soldiers every 120 days, adding that it would help Ukraine “scale-up their resistance”.
As part of the course, the government says the soldiers would learn “battle winning skills for the front line, as well as basic medical training, cyber-security and counter explosive tactics”.
No 10 said the leaders also discussed how the UK could “play a pivotal role in ending the blockade of grain”.
Mr Johnson said the purpose of his visit was “to send a clear and simple message to the Ukrainian people: the UK is with you, and we will be with you until you ultimately prevail”.
“Two months on from my last visit, the Ukrainian grit, determination and resilience is stronger than ever, and I know that unbreakable resolve will long outlive the vain ambitions of President Putin.”
The prime minister had been due to appear at a conference held by a group of his Tory MPs in Doncaster, but cancelled the visit to travel to the Ukrainian capital instead.
Leader of the group of MPs, Jake Berry, told the BBC that it was “not a snub” that the PM pulled out of the Northern Research Group conference.
He said: “He is prime minister of our United Kingdom. Things happen that mean that plans have to change.”
Chair of the Northern Research Group of Tory MPs Jake Berry has insisted that the prime minister pulling out of their conference in Doncaster is “not a snub”.
But clearly, it could have made for an uncomfortable meeting.
It’s just days until the by-election in Wakefield, which Labour are hoping to nab back from the Tories after their disgraced MP had to resign following his conviction for sexually assaulting a 15-year-old boy.
The group of Tory MPs had also been calling for regions in the north of England to be able to set their own tax rates – as well as cuts to personal taxes.
But No10 will argue that duty called and the prime minister’s visit to Ukraine superseded any domestic commitments, as the country continues to be torn by war.
Mr Johnson’s trip comes after visits to Ukraine by European leaders including French President Emmanuel Macron and German Chancellor Olaf Scholz.
Following their trip, the European Commission has backed Ukraine’s bid to be given candidacy status to join the EU – bringing it one step closer to joining the bloc.
