Kosovo's prime minister has called his Slovenian counterpart to ask for the release of a Kosovar opposition leader and former guerrilla commander detained on Wednesday on a war crimes warrant issued by Serbia.

The arrest of Ramush Haradinaj, who served briefly as prime minister of Kosovo in 2004-05, has infuriated Kosovars, many of whom consider him a hero for his role in fighting Serbian forces during the 1998-99 Kosovo war.

Detained en route through the ex-Yugoslav republic of Slovenia on Wednesday, Haradinaj was later released by police but his passport was retained and he was told to remain in the European Union country until a judge processes his case.

Lawmakers in Kosovo voted to ask the government to send a protest note to Slovenia.

Prime Minister Isa Mustafa spoke to his Slovenian counterpart, Miro Cerar, by phone, asking him "to allow Haradinaj to return to Kosovo as soon as possible", Mustafa's office said.

Haradinaj was tried and acquitted twice of war crimes at a United Nations court in The Hague. He frequently travels through Europe, including Slovenia, and it was unclear why police acted on the 2004 warrant on this occasion and not earlier.

The district court in Kranj, Slovenia, said it had not yet received an extradition request from Serbia, which is under pressure from the EU to improve ties with its former Kosovo province if it is to make progress towards membership of the bloc.

In a statement, the court said it might take days for a judge to decide whether or not Haradinaj should be allowed to leave Slovenia. Another former Kosovo prime minister, Agim Ceku, was arrested in Slovenia in 2003 on a Serbian warrant but later released.

"If this precedent continues, Serbia may issue arrest warrants for any citizen of Kosovo," Ardian Gjini, a member of Haradinaj's AAK party, told a news conference.

Kosovo and Slovenia otherwise enjoy good diplomatic and economic relations, with Ljubljana one of the biggest investors in the former Serbian province since it declared independence from Serbia in 2008.

"This unjustifiable act must not have any impact on our excellent relations with Slovenia," Kosovo Foreign Minister Hashim Thaci told parliament. "Slovenia is one of the more friendly nations we had in the most difficult of times."

Last week Naser Oric, a Bosnian Muslim commander during Bosnia's 1992-95 war, was arrested in Switzerland on a Serbian warrant for alleged war crimes and remains in custody.