Serbia returns remains of murdered Kosovo Albanians.

PRISTINA, Serbia and Montenegro, Dec 16 (Reuters) - Serbia was to return the remains on Thursday of some 50 Kosovo Albanians killed in 1999 and buried with hundreds of others in a mass grave just outside Belgrade.

The handover to Kosovo was to be the single largest between Serbia and the United Nations-run province since three mass graves were discovered in Serbia proper in 2001, containing more than 800 victims of the 1998-99 Kosovo war.

The existence of the graves was made public as the reformers who ousted former leader Slobodan Milosevic in 2000 tried to ready the country for his extradition to the U.N. war crimes tribunal in The Hague.

Around 350 identified remains have been returned since May 2003, out of a total of 836 bodies exhumed from all three sites.

The 50 to be returned on Thursday afternoon were among some 700 found in five large pits at the end of a firing range on the Batajnica police training ground just outside the Serbian capital. The bodies had been trucked in from Kosovo.

The province became a U.N. protectorate in 1999 after NATO bombing expelled Serb forces accused by Western powers of ruthless disregard for civilians in fighting a rebel insurgency.

An estimated 10,000 people died in the war. More than 3,000 are still missing, including 2,400 ethnic Albanians.

Kosovo's U.N. overseers say Serbia should speed up the handover of bodies for the sake of reconciliation.

"They can go faster," Jose-Pablo Baraybar, the Peruvian head of the U.N. Office on Missing Persons and Forensics in Pristina, said last week. "Why they aren't, they alone should explain."

Serbia is obliged to identify the bodies before returning them, a process Belgrade says is slow and complicated.

Baraybar said that despite post-mortems conducted by Serbia, the vast majority of bodies had been returned with cause of death unknown. Subsequent examinations by U.N. experts have established that 65 percent died of gunshot wounds.

Serbia says it is investigating what U.N. prosecutors says was a systematic operation to conceal evidence of atrocities in the Albanian-majority province, but has yet to charge anyone.

It is also refusing to hand over four army and police generals indicted by the tribunal of war crimes in Kosovo.

Kosovo remains formally a part of Serbia and Montenegro. The West says it will decide whether Albanians will get the independence they demand in negotiations expected to begin next year.