Europe's top human rights watchdog launched a probe Monday into Serb allegations that ethnic Albanian guerrillas kidnapped Serb civilians during Kosovo's war, removed their organs and sold the body parts on the black market.

Leading the probe is Dick Marty, a Swiss senator representing the Council of Europe. He is expected to meet top Serbian judiciary and war crimes officials during his two-day visit to Belgrade starting Monday.

Serbian officials say up to 500 Kosovo Serbs vanished without a trace during the 1998-99 war. They claim at least some of them may have had organs removed.

The Serb war crimes investigators say they have concrete proof that at least 10 people have been the victims of an international organ trafficking operation, but that many more may have been operated on in makeshift hospitals in neighboring Albania before being dumped in mass graves.

"Whatever the truth is, we are missing between 300 and 500 people and that is what those seeking justice must have in mind," Bruno Vekaric, spokesman for Serbia's war crimes prosecutors, said.

Ethnic Albanian officials have denied the claims. They said the allegations are part of Serbian propaganda against Kosovo's independence, declared last year with Western backing.

The allegations were first made public in a memoir last year by Carla Del Ponte, the former chief U.N. war crimes prosecutor. In "Madame Prosecutor," an account of her tenure as head of the International Criminal Tribunal for former Yugoslavia, Del Ponte said her office was tipped to possible organ trafficking.

The ethnic Albanian guerrillas fought Serbian troops loyal to late Serbian President Slobodan Milosevic in a conflict over ownership of Kosovo that claimed at least 10,000 lives. The bloodshed ended after NATO pummeled Serbia with airstrikes and sent in peacekeepers in June 1999. Kosovo's ethnic Albanian leaders declared independence from Serbia in February 2008.