Rishi Sunak has travelled to meet President Volodomyr Zelensky in Kyiv.
In remarks tweeted during his first visit to Ukraine as Prime Minister, Mr Sunak said: "Britain knows what it means to fight for freedom.
"We are with you all the way."
In a video released by No10, Mr Sunak is shown greeting Mr Zelensky, saying: "Mr President, it's really good to see you."
Mr Zelensky, gesturing to the rainy weather, joked: "You brought us England."
"Well there we go," Mr Sunak replied, telling him he'd brought a coat for later.
He added: "Your team have been fantastic, thank you for getting me here."
The video, backed with dramatic music, showed the pair shaking hands in front of Ukrainian flags, and Mr Sunak being introduced to figures in uniform before sitting down with Mr Zelensky for a bilateral discussion.
Downing Street said Mr Sunak found it "deeply humbling" to be in Kyiv.
During the meeting, No10 said, Mr Sunak confirmed a new package of air defence to protect protect Ukrainian civilians and national infrastructure from Russian strikes.
The £50 million package of defence aid comprises 125 anti-aircraft guns and technology to counter drones, including radars and anti-drone electronic warfare capability.
Earlier, the PM laid flowers at a memorial for the war dead and lit a candle at a memorial for victims of the Holodomor famine, before meeting first responders at a fire station, Downing Street said in a press release released to coincide with the visit.
They said: "The team of emergency responders described their harrowing work rescuing survivors from the rubble and fighting fires in the aftermath of Russian airstrikes and mortar attacks."
Mr Sunak also saw captured drones which have been used to target and bomb civilians in recent months.
The PM said: “I am proud of how the UK stood with Ukraine from the very beginning. And I am here today to say the UK and our allies will continue to stand with Ukraine, as it fights to end this barbarous war and deliver a just peace.
“While Ukraine’s armed forces succeed in pushing back Russian forces on the ground, civilians are being brutally bombarded from the air. We are today providing new air defence, including anti-aircraft guns, radar and anti-drone equipment, and stepping up humanitarian support for the cold, hard winter ahead.
“It is deeply humbling to be in Kyiv today and to have the opportunity to meet those who are doing so much, and paying so high a price, to defend the principles of sovereignty and democracy.”
