BRUSSELS (Reuters) - European Union officials said the door to Serbia and its former province Kosovo remained open if their relationship improved, pledging to review their bids to strengthen ties with the EU in the next six months.

Serbia hopes to start official negotiations to join the European Union and Kosovo wants to get working on a trade and financial assistance deal. But the EU is concerned about continued frictions between the two Balkan foes, which fought a war in 1998-99. Kosovo declared independence in 2008, which Serbia does not recognize.

"A visible and sustainable improvement in relations between Serbia and Kosovo is needed so that both can continue on their respective European paths," European affairs ministers from the EU's 27 members, most of which recognize Kosovo, said in a statement.

Speaking after late night talks to assess the progress of countries hoping to join the EU, they said a decision over how to proceed with Serbia and Kosovo could be taken after a review in the next six months.

Diplomats said some EU members wanted to offer the two encouragement after recent signs of progress. Serbia and Kosovo on Monday began to share control of two border crossings for the first time since the war. Two more out of a total of six are due to open by December 31.

"The idea was to give positive reaction without immediately... setting the date for negotiations," one EU diplomat said, speaking on condition of anonymity.

The northern part of Kosovo that borders on Serbia is still largely beyond the control of the Kosovo government and populated by minority Serbs who refuse to accept Kosovo as a sovereign state.

NATO intervened in 1999 with 78 days of air strikes to drive Serb forces from Kosovo and halt the killing and expulsion of ethnic Albanians.

More than 90 countries now recognize Kosovo as independent, including the United States.

(Reporting by Justyna Pawlak; Editing by Sonya Hepinstall)