Rishi Sunak and Joe Biden stressed their “unwavering support” for Ukraine, with the president saying the US will help Kyiv for “as long as it takes”.
The two leaders emphasised the need for long-term security arrangements for the war-torn nation as deterrence against aggression from Russian president Vladimir Putin.
At a joint press conference at the White House on Thursday, Mr Biden was asked about hesitation about more funding for Ukraine among some lawmakers in the rival Republican Party.
“And I believe that that support will be real, even though you hear some voices today on Capitol Hill about whether or not we should continue to support Ukraine and for how long we should support them.”
He said Moscow would not halt its invasion if the west was not supporting Kyiv.
Mr Sunak said: “I agree with what the president has said – it’s about deterrence.
“President Putin will be thinking that he can wait us out, that the alliance will tire, will get fatigued and will give up. Now, that is not the case.
“And the more we can put in place support for Ukraine, not just in the here and now, but support that will last for a time and for years to come, I think it sends a strong signal to him that there is no point in trying to wait us out, we’re not going anywhere. We will be here for as long as it takes.”
Mr Biden also thanked Mr Sunak for his “strong, strong leadership” over the war in Ukraine.
The Prime Minister in turn praised Washington, saying: “The UK is proud of our contribution including providing tanks, long range weapons and training Ukrainian soldiers – but let no-one doubt US leadership and resources are the decisive contribution allowing the forces of democracy and freedom to prevail.”
The two leaders have “unwavering support of the people of Ukraine”, the US president said.
Mr Biden and Mr Sunak also discussed helping Ukraine in their bilateral meeting, according to a read-out.
A Downing Street spokesperson said: “They agreed that the UK and US continue to have an important role to play in bringing stability in regions throughout the world, and countering state and non-state hostile activity. This includes supporting Ukraine in their fight against Putin’s illegal invasion.”