US President Donald Trump has sent a letter to Kosovo counterpart Hashim Thaci urging him to do everything to reach a longstanding deal with Serbia two decades after their war ended, Reuters reported quoting Thaci’s official website.

“Failure to capitalise on this unique opportunity would be a tragic setback, as another chance for a comprehensive peace is unlikely to occur again soon,” says the text of the letter posted on Thaci’s website on Tuesday.

“The United States has invested heavily in the success of Kosovo as an independent, sovereign state,” it said.

In Washington, the White House had no comment. There was no immediate comment from the U.S. Embassy in Kosovo’s capital Pristina.