US President Donald Trump has revealed his awaited "major statement" on Russia, threatening Moscow with "very very severe" tariffs unless a deal to end the war with Ukraine is reached within 50 days.

"We're going to be doing secondary tariffs. If we don't have a deal in 50 days, it's very simple. And they'll be at 100%, and that's the way it is," Trump told reporters at the White House.

Trump made the announcement as he sat side-by-side with NATO Secretary-General Mark Rutte in the Oval Office, saying he was very unhappy with Russia.

"I use trade for a lot of things," he added. "But it's great for settling wars."