Rishi Sunak said he was "humbled" to be in Ukraine, as the Prime Minister held his first face-to-face meeting with President Volodymyr Zelensky.
The visit came as the Prime Minister, in office only a matter of weeks, pledged continued British support to the war-torn country in the fight against Russia.
Mr Sunak's arrival was accompanied by the announcement of a £50million package of defence aid comprising 125 anti-aircraft guns and technology to help Ukraine counter Iranian-supplied drones, including radars and anti-drone technology.
The visit came after a week that saw tensions between the West and Russia ratchet up, after a missile hit a Polish village close to the Ukrainian border and sparked concerns Russia had struck a Nato member state.
Britain knows what it means to fight for freedom.
We are with you all the way @ZelenskyyUa 🇺🇦🇬🇧
Британія знає, що означає боротися за свободу.
Ми з вами до кінця @ZelenskyyUa 🇺🇦🇬🇧 pic.twitter.com/HsL8s4Ibqa
— Rishi Sunak (@RishiSunak) November 19, 2022
Tensions have eased since by the likely attribution of the hit to Ukrainian defence forces, but Russian missile strikes on the country have shown no sign of relenting.
In Kyiv, Mr Sunak laid flowers at a memorial for the war dead and lit a candle at a memorial for victims of the Holodomor famine, before meeting emergency personnel at a fire station.
He said it was "deeply humbling" to be in Kyiv, and he said he was "proud" of how the UK had backed Ukraine since the beginning of the war.
"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," he said.
"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."
During the visit, Mr Sunak also confirmed £12million for the World Food Programme’s response to Ukraine, as well as £4million for the International Organisation for Migration, ahead of what is expected to be a long winter for the country.
Downing Street said the funding would help provide generators, shelter, water repairs and mobile health clinics, with the UK also sending tens of thousands of extreme cold winter kits for Ukrainian troops.