The three-month NATO operation began in March 1999 and continued until Serbian leader Slobodan Milosevic withdrew his troops from Kosovo, where they had intervened against separatists.

The 1998-1999 independence war in Kosovo claimed more than 13,000 lives, mostly ethnic Albanians, and led to a refugee crisis in which hundreds of thousands of people left their homes.

The former Serbian province, comprised mainly of ethnic Albanians, was put under United Nations administration after Serbian troops pulled out.

Kosovo unilaterally declared independence in 2008 but Serbia refuses to recognize it.

"NATO's intervention brought an end to the Milosevic regime's campaign of ethnic cleansing, which left thousands dead and over one million civilians displaced," a White House statement said.

"To pay respects to those victims, Kosovo and Serbia must recommit to a peaceful and prosperous future for all of their citizens. This requires putting aside extremist rhetoric and historical revisionism."

Long-running negotiations to normalize ties have halted in recent months after a series of diplomatic clashes.

The White House called for dialogue between the neighbors "aimed at normalizing relations" and urged Pristina and Belgrade to "redouble efforts to work toward a comprehensive agreement centered on mutual recognition and immediately remove barriers to progress."

During the air campaign by the North Atlantic Treaty Organization, a political and military alliance based in Brussels, warplanes struck dozens of military targets, as well as infrastructure such as bridges, railways and the electrical grid.

The number of civilian victims has not been officially established, and ranges from 500 according to Human Rights Watch to 2,500, according to Serbian officials.