U.S. President Donald Trump has urged Serbia and Kosovo to secure a “historic” deal that would bring “long-sought” peace to the Balkan region — a sign that Washington is willing to take a more active role in mediating between the two wartime foes, AP reported.

The office of Serbian President Aleksandar Vucic on Thursday published a letter he received from Trump, in which the U.S. president says “I encourage you to seize this moment and to exercise the political leadership in making decisions needed to balance the interests of both countries.”

“It would be extremely regrettable to miss this unique opportunity for peace, security, and economic growth,” Trump said.

Kosovo President Hashim Thaci received a similar letter from Trump.

Trump said in the letter that the U.S. stands ready to assist and that he is looking forward to hosting the two presidents in the White House “to celebrate what would be an historic accord.”

“An agreement between Serbia and Kosovo is within reach,” Trump said.