PRISTINA, Kosovo — The European Union mission in Kosovo has launched a preliminary investigation into allegations that Kosovo Prime Minister Hashim Thaci once led a ring trafficking in human organs, an official said Friday.

The EU mission, known as EULEX, cited a Council of Europe report in December that suggested Thaci was once the “boss” of a criminal underworld behind the alleged organ trade during the 1998-99 war against Serbia, EU mission spokesman Karin Limdal said. Thaci has denied that.

The mission said it took those allegations “very seriously” and was “ready, willing and able to assume responsibility” for a judicial process, Limdal said in a statement.

The report alleges that civilian detainees of the now-disbanded Kosovo Liberation Army rebels were shot dead to sell their kidneys on the black market.

Thaci, who was a leader of the guerrilla group in the war, has denied the allegations and claimed they were aimed at damaging Kosovo’s image internationally. He and other Kosovo leaders have called for an investigation.

EU officials appealed for those who could help the investigation and present evidence.

“As a rule of law mission, we work on the basis of fact and evidence. Without evidence, prosecutions cannot take place,” the EU statement said.