The UN Security Council has condemned "in the strongest terms" attacks by protesters against embassies in the Serbian capital, Belgrade.
Police were not guarding the US embassy at the time
Serbs angered by Western support for Kosovo's declaration of independence set fire to parts of the US embassy.
An unidentified charred body was found inside. The US has denounced the "thugs" who carried out the attack.
The embassies of the UK, Germany and Turkey and businesses with Western links were also targeted by protesters.
The violence followed a rally against Kosovo's secession.
Most Serbs regard Kosovo as their religious and cultural heartland.
"As long as we live, Kosovo is Serbia," Prime Minister Vojislav Kostunica told the rally.
Afterwards, about 1,000 protesters smashed their way into the US embassy, throwing flares through a window, while others scaled walls to rip down the US flag.
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 the 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.
In New York, the 15-member UN Security Council issued a unanimous statement: "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.
"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.
Mr McCormack said the US had warned Mr Kostunica and his foreign minister that it would hold them personally responsible for further damage.
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 said he was concerned about reports that Serbian government officials had helped stir the attack.
"We have seen a lot of disturbing reports about statements by Serbian government officials, even including a minister, about incitement to violence. This has to cease."
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."
European Union spokeswoman Cristina Gallach said the violence was "totally unacceptable".
She urged all involved to show "restraint and responsibility".
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."
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.