US President Donald Trump threatened on Friday to impose additional tariffs on Russia if it doesn't make room for a ceasefire by the end of April.

US President Donald Trump is disappointed with the lack of correspondence between Russia and Ukraine in his plan to establish peace between the parties, according to a spokesman in Washington.

"As these negotiations are ongoing, I'm obviously not going to defer to the president or his team, but I think the president has made it very clear that he is continually frustrated with both sides of this conflict and wants to see this fight end," he said.

"He wants the war to end. We believe that we have leverage in negotiating an agreement, a peace agreement, and we will use that leverage, and the president is determined to see it through," the spokesman added.