US Secretary of State Hillary Clinton leaves Sunday on a trip to the Balkans where she will urge Serbia and Kosovo to overcome bitter differences and integrate fully with the EU and NATO.

Clinton, who will first hold talks Monday in Algeria on the political crisis in Mali, will be joined by EU foreign policy chief Catherine Ashton for part of her Balkans visit with the two expected to push for greater integration.

The top US diplomat will first travel to Bosnia, the only Balkans country yet to apply for an EU membership, but which wants to submit a formal application later this year. It remains deeply divided along ethnic lines however, with Bosnian Serb leaders threatening to break away.

"We have not been shy about saying and making clear that we are disappointed, that the leaders of Bosnia Herzegovina have not put the interests of the country first (and) instead have promoted narrow ethnic group parties for personal agendas," a State Department spokesman said, asking to remain anonymous.

"I think the message from Secretary Clinton and High Representative Ashton will be that... we would like to see it becoming a member of European Union/NATO, but we really need the necessary reforms."

Clinton last visited Bosnia, as well as Serbia and Kosovo -- countries born out of the 1990s collapse of the former Yugoslavia -- in 2010.

Without Ashton, she will end her visit in Albania and Croatia, which both joined NATO in 2009. Of the six ex-Yugoslav nations, only Slovenia has so far joined the European Union in 2004, while Croatia is due to become a member in July.

Zagreb is an example for the rest of the region, the State Department official said.

"This an important thing we are stressing, this is something the United States had long supported, but it is also a message to the region," the official said.

"Yes, it is difficult, yes, reforms are hard, yes the road is long to European Union membership, but if you do the right things and you do what has been agreed, you actually cross the finish line."

That message is primarily aimed at bitter foes, Serbia and Kosovo.

Talks between Belgrade and Pristina, launched in March 2011 under the auspices of the EU, were suspended after May's elections in Serbia won by nationalist forces.

Earlier this month, Ashton chaired a top-level meeting with the prime ministers of the two nations, as Belgrade refuses to recognize Pristina's unilateral declaration of independence from Serbia in 2008.

The EU and Washington are calling on the two to formally resume direct talks with the aim of hopefully winning a date, perhaps as early as December, to start accession talks for joining the European Union.

Serbia has said that while it is happy to resume talks with Kosovo, the Albanian-majority region remains for them a breakaway province and its independence is unacceptable.

Foreign Minister Ivan Mrkic told AFP in a written statement that "the visit will be a good opportunity to have a thorough exchange of opinions how Serbia and US will further strengthen their relations, as well as prospectives of strengthening peace and development in our region."

Serbian President Tomislav Nikolic said last week that Belgrade was "ready to make a lot of compromises in order to solve (the) Kosovo issue, but it would expect (the) ethnic Albanian side to do the same."

Kosovo declared independence a decade after the brutal conflict between Kosovan rebels and Serb forces was ended by NATO's 1999 intervention.

Hillary Clinton's husband, Bill, was the US president at the time, and the couple are treated as heroes in Kosovo. A statue to Bill Clinton has been erected on one of the main streets in the capital, Pristina.

In Bosnia, Clinton and Ashton will insist that the 1995 Dayton accords which ended the war and were negotiated by the Clinton administration, are here to stay.

Bosnian Serb leader Milorad Dodik has repeatedly referred to Bosnia's Serb entity, the Republika Srpska, as a state and warned it could break away. He says the RS, which along with the Muslim-Croat Federation makes up post-war Bosnia, should also negotiate separately on EU entry.

State Department spokeswoman Victoria Nuland said Clinton and Ashton will "stress the immutability of the international community's commitment to the Dayton Peace Accords."

Last week Britain's Foreign Secretary William Hague also warned Bosnian leaders to overcome their ethnic divisions if they want to bring the country closer to the European Union and NATO.