The European Union has called on Serbia to protect foreign embassies following a day of violence in Belgrade.
Police were not guarding the US embassy at the time
Serbs angered by Western support for Kosovo's secession set fire to the US embassy and attacked the UK, German, Croatian, Belgian and Turkish embassies
The UN Security Council has condemned the attacks, while the US described those responsible as "thugs".
EU foreign policy chief Javier Solana said continued violence would harm efforts to improve ties.
"The embassies have to be protected, and that is the obligation of the country," Mr Solana told reporters when arriving for an EU meeting in Slovenia.
"Things will have to calm down before we can recuperate the climate that would allow for any contact to move on the Stabilisation and Association Agreement."
That agreement, a preliminary deal on EU-Serbia links, was reached last year, but the EU has said it will not sign it until Belgrade co-operates fully with the UN war crimes tribunal for the former Yugoslavia.
The EU was ready to sign an interim trade deal with Belgrade, but Serbian Prime Minister Vojislav Kostunica blocked the move earlier this month.
European Union spokeswoman Cristina Gallach said the violence in the Serbian capital - which followed a rally against Kosovo's secession - was "totally unacceptable".
She urged all involved to show "restraint and responsibility".
Mr Kostunica has condemned the "brutal violence" that saw about 1,000 protesters smash their way into the US embassy, throwing flares through a window.
At the time there appeared to be no police protecting the embassy, but riot police later intervened, firing tear gas to disperse the crowd.
The fires raged for half an hour, and when firemen finally managed to get inside the building they found a charred body, which has not been identified.
US officials said all embassy staff of US nationality had been accounted for, and reports suggest the body may be that of one of the intruders.
Photographer Joachim Heinemann in Belgrade says the streets of the city are now calm, with the clean up operation well under way.
He told the BBC News website that shops selling Western sportswear brands had been targeted by rioters, as had American burger chain McDonald's.
"Shops selling Nike and other American brands had their windows smashed, while the city's McDonald's branches have been systematically destroyed," he said.
In New York, the 15-member UN Security Council issued a unanimous statement condemning the violence: "The members of the Security Council condemn in the strongest terms the mob attacks against embassies in Belgrade, which have resulted in damage to embassy premises and have endangered diplomatic personnel."
It went on to welcome steps by the Serbian authorities to restore order.
White House spokesman Dana Perino employed less diplomatic language.
The main rally outside parliament was peaceful
"Our embassy was attacked by thugs," she told reporters on US presidential plane Air Force One as President George W Bush and Secretary of State Condoleezza Rice returned from a trip to Africa.
She added: "We have made known to the Serbian government our concern and displeasure that their police force did not prevent this incident."
US state department spokesman Sean McCormack said a formal protest had been lodged with Serbia.
"The message was very clear - that the situation was intolerable, they needed to act immediately to provide the adequate security forces so that our embassy compound and our personnel were not under attack," he told reporters.
Population about two million
Majority ethnic Albanian; 10% Serb
Under UN control since Nato drove out Serb forces in 1999
2,000-strong EU staff to take over from UN after independence
Recognised by US, UK, Germany, Italy and France
Not recognised by Russia, Spain, Slovakia, Cyprus
Nato to stay to provide security
He added: "We received assurances from Prime Minister Kostunica that there would not be a repeat of this episode and we will hold him to that."
British Foreign Secretary David Miliband said damage to the British embassy building was "limited", and embassy staff were safe.
"While the Kosovo issue raises strong feelings in Serbia, no cause can justify such displays of violence," he said.
"We have made clear to the Serbian government that we expect them to fully uphold their obligations to protect our embassy and other diplomatic premises in Belgrade."
Serbia said it regretted what it called acts of isolated vandals who did not represent the nation.
"The acts that were committed are absolutely unacceptable, absolutely regrettable," Foreign Minister Vuk Jeremic told Reuters news agency.
"They hurt Serbia's image abroad."
Most Serbs regard Kosovo as their religious and cultural heartland.
Serbia, supported by Russia and China, says Kosovo's independence declaration on Sunday violates international law.
Mr Kostunica has recalled Serbia's ambassador to the US.
In the coming weeks, a 2,000-strong EU mission will be deployed to help Kosovo develop its police force and judiciary.
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