U.S. President Donald Trump said on Sunday that individual European leaders would visit the United States on Monday or Tuesday to discuss how to resolve the Russia-Ukraine conflict.

Speaking to reporters after returning from the U.S. Open in New York City, Trump also said he would speak to Russian President Vladimir Putin soon.

"Certain European leaders are coming over to our country on Monday or Tuesday individually," Trump said. It was not clear to whom Trump was referring, and the White House did not immediately respond to a request for further details.

Trump added that he was "not happy" about the status of the Russia-Ukraine conflict, after reporters asked about a massive Russian air assault overnight on Sunday that Ukrainian officials said had set the main government building in Kyiv on fire. But he again expressed confidence that the conflict would soon be settled.

"The Russia-Ukraine situation, we're going to get it done," Trump said.

Trump also suggested on Sunday he is ready to move to a second phase of sanctioning Russia, the closest he has come to suggesting he is on the verge of ramping up sanctions against Moscow or its oil buyers.

(With input from Reuters)