Keir Starmer urged world leaders to "redouble their efforts" to support Ukraine during a visit to Poland, just days before Donald Trump's return to the US presidency threatens to jeopardize international unity on the issue.

Starmer, speaking alongside Polish Prime Minister Donald Tusk after the two discussed the proposed defense-focused treaty, dodged questions about Trump's possible impact but insisted the only way forward was " Peace on Ukrainian terms”.

He also faced a slightly awkward moment when Tusk, who chaired the European Council for much of the Brexit period, said it was "my dream" for Britain to return to the EU - "not Brexit, but "Brexit" - this is definitely not the policy of the British government.

Starmer, who held talks with Volodymyr Zelensky on Thursday during his first official visit to Ukraine since taking office, said defense and security issues dominated the Warsaw talks.

"As Europe's two largest military powers, our troops serve together to defend the Eastern Flank, and we have an unbreakable commitment to both NATO and Ukraine," he said.

“We all know that the path to a just and lasting peace depends on strength, the strength to ensure peace on Ukraine’s terms, and the strength to sustain it.

"We will work with our allies to do this and intensify our efforts to put Ukraine in its strongest position now and ensure that it can defend itself and deter Russia in the future."

Trump expressed a variety of views on Russia's invasion of Ukraine, but focused on his belief that he could broker a quick peace.

Trump's idea may not satisfy Ukraine's demand that it not lose any territory, and there are significant questions about whether the United States will maintain full military and diplomatic support.

Asked about one of the factors, Trump saying it would be "crazy" to allow Ukraine to use long-range missiles against Russian targets, Starmer dodged the question slightly and said he would not provide "ongoing commentary on other allies" .

But he added: "I think it's absolutely clear that the United States is critical to the defense of Ukraine."

In Trump's diplomatic blitz ahead of Monday's inauguration, Starmer traveled to Warsaw after promising Zelensky a "closer than ever" partnership in Kyiv, and Britain and Ukraine signed a deal to deepen the Agreement on defense relations.

Earlier on Friday, Starmer visited the Auschwitz Memorial and Museum accompanied by his Jewish wife Victoria.

Speaking later in Warsaw, he explained the impact of visiting the remains of the Nazi death camps: "I stood on those tracks and looked at the shoes, the suitcases piled high, the hair, the teeth, all of which had come from those tracks. Taken down." Killed by the Nazis.

"I will never forget what I saw today. I will never forget my responsibility to pass this on to the next generation, to remember, learn and respect our solemn promise: Never again."