US President Donald Trump and British Prime Minister Keir Starmer met at the White House on Thursday for their first in-person talks since the Republican leader upended US policy on Ukraine, the Middle East, and global trade.

Starmer is the second European leader to meet Trump this week after French President Emmanuel Macron came to the White House on Monday for a friendly encounter that nonetheless displayed stark differences about Russia's war with Ukraine and the US push for a quick ceasefire.

The British leader arrived at the White House on Thursday afternoon, where he was greeted by the president at the West Wing entrance ahead of meetings and a press conference.

Asked if the US could get a peace deal on Ukraine, Trump said that a peace deal needs to be struck soon between Ukraine and Russia or it may not happen "at all."

"I think we've made a lot of progress and I think it's moving along pretty rapidly," Trump said. "It'll either be fairly soon or it won't be at all."

Trump said he trusted Russia's Vladimir Putin to stick to any Ukraine ceasefire, as Starmer tried to win security guarantees for Kyiv.

He walked back a remark about Ukrainian President Volodymyr Zelensky being a dictator that alarmed European capitals, saying, "Did I say that?"

Trump told reporters as he sat alongside Starmer in the Oval Office when asked about Putin, "I think he'll keep his word."

He added that Britain can "take care of themselves, but if they need help, I'll always be with the British."

The British premier told Trump at the White House that he wanted to "work with you to make sure that the peace deal is enduring" but that it was also a "deal that nobody breaches."

Trump also expressed hope for a trade deal to be struck between both countries.

"We're going to have a great trade agreement, one way or the other," Trump told reporters. "We're going to end up with a very good trade agreement for both countries, and we're working on that as we speak."

Trump will host Zelensky at the White House on Friday, where the two leaders are expected to sign a deal giving Washington access to Ukraine's rare minerals, which Trump has demanded as payback for US military aid. (Agencies)