FILE - Workers hang Serbian flags in the northern, Serb-dominated part of ethnically divided town of Mitrovica, Kosovo, Dec. 14, 2018.
FILE - Workers hang Serbian flags in the northern, Serb-dominated part of ethnically divided town of Mitrovica, Kosovo, Dec. 14, 2018.

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.

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.

Security Council Vucic Thaci
Serbian President Aleksandar Vucic and Kosovo President Hashim Thaci

Kosovo President Hashim Thaci received a similar letter from Trump.

Kosovo broke away from Serbia after a 1998-99 war and NATO intervention that stopped a bloody Serbian crackdown against Kosovo Albanian separatists. In 2008, Kosovo unilaterally declared independence which is recognized by the U.S. and most of the West, but not by Serbia and its allies Russia and China.

Belgrade has warned of an armed intervention after Kosovo made moves toward creating an army earlier this month.

Serbia and Kosovo have been conducting EU-mediated talks aimed at normalizing relations, but with little headway. Trump's letter appears to be an attempt by Washington to take a role in the mediation after showing little interest in tensions in the Balkans since he assumed the presidency.

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.