US President Donald Trump deepened his rift with Europe in an interview with Politico published Tuesday, calling the continent"weak" and "decaying" over immigration and Ukraine.

Trump doubled down on his extraordinary recent criticisms of a region that Washington has long counted as a key ally, recycling far-right tropes about civilisational decline in Europe.

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

The 79-year-old billionaire, whose political rise to power was built on inflammatory language about migration, echoed far-right talking points as he said that Europe's policies were a"disaster".

"They're coming in from all parts of the world," Trump said."But they want to be politically correct, and they don't want to send them back to where they came from."

Trump's broadside comes days after his administration's new national security strategy sparked alarm by calling for the cultivation of "resistance" in the EU against liberal migration policies.

Asked if European countries would not remain US allies if they failed to embrace his administration's policies on the issue, Trump replied that"it depends".

"I think they're weak, but they also want to be so politically correct," Trump said.

Swipe at London's Muslim mayor

He listed countries including Britain, France, Germany, Poland and Sweden that he said were being"destroyed" by migration, and launched a new attack on the"horrible, vicious, disgusting" Sadiq Khan, London's first Muslim mayor.

Trump also brushed off the fact that the Kremlin had hailed the new US national security strategy as being in line with its own views.

"I think he (Putin) would like to see a weak Europe, and to be honest with you, he's getting that. That has nothing to do with me," he said.

Trump also criticised Europe's role in resolving the war between Russia and Ukraine, saying:"They talk but they don't produce. And the war just keeps going on and on."