"It is because of President (Volodymyr) Zelensky's resolute leadership and the invincible heroism and courage of the Ukrainian people that (Vladimir) Putin's monstrous aims are being thwarted," he said after meeting Zelensky, according to a Downing Street statement.

Johnson set out extra military aid of 120 armoured vehicles and new anti-ship missile systems, "to support Ukraine in this crucial phase while Russia's illegal assault continues", the statement said.

That is on top of UK aid announced Friday of additional Starstreak anti-aircraft missiles and another 800 anti-tank missiles, along with "loitering" drones for "precision strikes" against the Russians.

As world powers held a fundraising round for Ukraine, Johnson also promised an extra $500 million via the World Bank.

Johnson said it had been a "privilege" to meet Zelensky in person on his surprise visit, which was not pre-announced in London.

"Ukraine has defied the odds and pushed back Russian forces from the gates of Kyiv, achieving the greatest feat of arms of the 21st century," he said.

"I made clear today that the United Kingdom stands unwaveringly with them in this ongoing fight, and we are in it for the long run."