Chequers, UK - President Donald Trump said Thursday that Vladimir Putin has "really let me down" over the war in Ukraine, as British Prime Minister Keir Starmer called for more pressure on the Russian leader.

At a joint press conference with Starmer on a UK state visit, Trump said failing to end the conflict was his "biggest disappointment" but did not set out his next steps against Putin.

Trump has urged Western allies to stop buying Russian oil to force Putin to the negotiating table, but has held back from taking any further unilateral actions against Moscow.

The US president hosted Putin in Alaska for talks in August and has unsettled allies with his repeated pivots towards the Russian leader.

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"The one that I thought would be easiest (of the conflicts he is trying to end) would be because of my relationship with President Putin, but he's let me down," Trump said. "He's really let me down."

Trump insisted that, however, "Russia and Ukraine will get done."

The British premier has positioned himself as a bridge between Trump and European allies, particularly on the war in Ukraine, in a bid to secure more commitments for Kyiv from the US leader.

He was careful not to push Trump directly, but said that he and the US president had discussed together how to "decisively increase" pressure on Putin.

"We have to put extra pressure on Putin, and it's only when the president has put pressure on Putin that he's actually shown any inclination to move," Starmer said. "So we have to ramp that pressure up."