Serbia's troubled relations with Kosovo will force it to wait longer to start membership talks with European Union, EU diplomats said on Tuesday.

In its annual report Wednesday on countries lining up to join the EU, the European Commission is expected to say the former Yugoslav state has not done enough to mend ties with its former province since becoming a membership candidate in March.

Serbia has made great strides to try to shed its reputation as a pariah of the Balkans, linked to its central role in the bloody wars that tore through the peninsula after the collapse of Yugoslavia.

It reformed its economy and institutions, and arrested all war crime suspects. But tensions with Kosovo, which split from Belgrade in 2008, are holding it back from its EU path.

"It will not get a recommendation yet, it's not in the cards," one EU diplomat said, speaking on condition of anonymity.

Serbia lost control over Kosovo in 1999 after an 11-week NATO air war to halt the killing and expulsion of ethnic Albanians by Serb forces fighting a two-year counter-insurgency war under late strongman Slobodan Milosevic.

More than 90 countries, including the United States and 22 of the EU's 27 members, have recognized the impoverished, landlocked country as a sovereign state.

Belgrade has vowed never to do so, but is under pressure from the EU to improve cooperation with Pristina in a range of fields, ranging from trade, to state institutions and security.

Serbia is not alone in the Balkans in struggling to catch up with other former Soviet bloc states in their European integration. Macedonia's progress is stuck because of a row with Greece over its name and Montenegro has only won approval to start entry talks this year. Bosnia and Albania are even further behind.

Croatia, however, is scheduled to join in July 2013, and Slovenia is already a member not only of the EU, but the euro.

Public sentiment in the European Union is lukewarm towards further expansion, with citizens focusing on their own economic difficulties and governments scrambling to find a way out of a debt crisis.

A spat with Romania, one of the EU's two most recent members, over a failed power grab by Prime Minister Victor Ponta has highlighted wariness within the bloc over admitting countries where democratic reforms have not gone far enough.

"It's not in the interest of a candidate country to join the European Union and only then face the job not being done properly there," an EU official said.

(Additional reporting and writing by Justyna Pawlak; Editing by Alison Williams)