US President Donald Trump blasted Europe as "decaying" and "weak" on immigration and Ukraine in an interview published on Tuesday, deepening a rift between the United States and some of its oldest allies.

Speaking to Politico, Trump also called on war-battered Ukraine's President Volodymyr Zelensky to hold elections despite Russia's assault and said that Moscow had the "upper hand."

Trump's comments doubled down on extraordinary criticism of top US partners in his administration's new national security strategy last week, which recycled far-right tropes about civilisational "erasure" in Europe.

"Most European nations, they're decaying," Trump told Politico in the interview, conducted on Monday.

The 79-year-old billionaire, whose political rise to power was built on inflammatory language about migrants, said that Europe's policies on migrants were a "disaster."

"They want to be politically correct, and it makes them weak. That's what makes them weak," Trump said, adding that there were "some real stupid ones" among Europe's leaders.

Trump also criticised European nations over Ukraine, amid growing differences over a US plan to end the war that many in Europe fear will force Kyiv to hand over territory to Russia, which launched a full-scale attack on the country in 2022.

"NATO calls me daddy," Trump said, referring to comments by the military alliance's leader Mark Rutte at a summit in June when leaders backed Trump's call to raise defence spending.

But he added: "They talk but they don't produce. And the war just keeps going on and on." (AFP)