Belgrade has warned Sofia that its decision to release Kosovo's former prime minister two days after he was detained on an international arrest warrant issued by Serbia for alleged war crimes, will hurt bilateral relations.

"The ruling of Bulgaria's justice system will not help the good relations between the two countries," Interior Minister Ivica Dacic commented in Moscow, where he is paying an official visit.

According to Dacic there is political pressure over Interpol, which, according to him, hampers police cooperation and the fight against crimes and terror.

Ceku, a former rebel commander who was prime minister of Kosovo between 2006 and 2008, is wanted in Serbia for allegedly committing war crimes during the 1998-1999 fighting in Kosovo between ethnic Albanian rebels and Serb government forces.

Ceku was arrested while entering Bulgaria from Macedonia late Tuesday and on the next day Serbia filed an extradition request with the Bulgarian authorities.

Ceku was traveling to Bulgaria at the invitation of the Atlantic Club, a non-governmental organization, to lobby for NATO membership, the club's honorary president and Bulgaria's former Foreign Minister Solomon Pasi said.

Last month Ceku was expelled from Colombia, after Serbia urged his arrest, but later he transited France en route to the Balkans.

He had previously been detained in 2006 in Slovenia and Hungary.

Kosovo declared independence in February 2008. Ethnic Albanians form the overwhelming majority of the population.

The US and more than 50 other countries have recognised its independence, but more than 100 have not, including Serbia and Russia.