BERLIN (AP) — U.S. diplomats on Monday oversaw the signing of an agreement between Serbia and Kosovo on the resumption of commercial flights intended to help improve economic ties between the Balkan nations.

Serbia and Kosovo have remained uneasy neighbors ever since their 1998-99 war that claimed more than 10,000 lives and left over 1 million people homeless.

Serbia continues to consider Kosovo part of its territory, although its independence has been recognized by about 100 countries, including the United States.

U.S. National Security Adviser Robert C. O'Brien said the agreement signed in Berlin between representatives of the two countries marked a “historic deal," calling commercial air links the “lifeblood of a modern economy.”

O'Brien said there have been no commercial flights between Kosovo and Serbia in 21 years. Currently, it takes over five hours to travel from Belgrade to Pristina overland.

The agreement was inked by Milun Trivunac, state secretary of Serbia's Ministry of Economy, and Eset Berisha, director general of the Civil Aviation Authority of Kosovo, at the U.S. Embassy in Berlin. U.S. Ambassador Richard Grenell also serves as President Donald Trump's special envoy for Serbia and Kosovo.

The 25-minute flight between Belgrade and Pristina will be operated by Lufthansa's budget carrier Eurowings, which already has an aircraft stationed in the Kosovo capital.

Michael Knitter, the chief operating officer of Eurowings, said the flights will begin after further regulatory hurdles have been removed by both countries.