Several hundred protesters have attacked the US and other embassies in Serbia's capital in anger at Western support for Kosovo's independence.
Police were not guarding the embassy at the time
Protesters broke into the US compound in Belgrade and briefly set part of the embassy alight. Firemen later found an unidentified charred body inside.
Other embassies, including the UK's, were also targeted. The UN Security Council condemned the attacks.
The violence followed a peaceful rally by at least 150,000 people in the city.
Most Serbs regard Kosovo as their religious and cultural heartland.
Serbian Prime Minister Vojislav Kostunica delivered an impassioned speech condemning the territory's secession.
"As long as we live, Kosovo is Serbia. Kosovo belongs to the Serbian people," he told the flag-waving crowd.
Later about 1,000 protesters smashed their way into the US embassy, throwing flares through the 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.
The fires raged for half an hour, and when firemen finally managed to get inside the building they found a charred body.
The body has not been identified, though US officials said all embassy staff of US nationality had been accounted for.
The main rally outside parliament was peaceful
White House spokesman Dana Perino said the embassy had been "attacked by thugs" and that Serbian police had not done enough to stop them.
State Department spokesman Sean McCormack said the US had warned Mr Kostunica and his foreign minister that it would hold them personally responsible for further damage.
Mr McCormack added that the protesters had entered the chancellery but had not breached the embassy's secure area.
Smaller groups later targeted the Croatian, Turkish and British embassies but were beaten back.
In New York, the UN Security Council condemned what it called "mob attacks" on US and other embassies in Belgrade.
In a unanimous statement, the council recalled the inviolability of diplomatic missions under international law, but welcomed steps by Serbian authorities to restore order.
Serbian President Boris Tadic appealed for calm.
"This only keeps Kosovo distant from Serbia," he said.
Serbia, supported by Russia and China, says Kosovo's Sunday declaration violates international law.
During Thursday's rally, ultra-nationalist leader Tomislav Nikolic accused the US and EU of trying to steal Kosovo.
"Hitler could not take it away from us, and neither will today's [Western powers]."
After the speeches, the crowd marched to the city's biggest church, the Temple of Saint Sava.
Thick, black smoke had also earlier billowed from the crossing point at Merdare, 50km (30 miles) north-east of Kosovo's capital Pristina, as Serb army reservists tried to enter Kosovo.
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
"We are here in support of the Serbs who still live in Kosovo," Dejan Milosevic, one of the organisers, told the Associated Press news agency.
The Kosovo police, backed by Czech troops from the Nato-led peacekeeping force, put a steel barrier across the road and were able to hold their line.
Protest rallies were also held in the Bosnian Serb republic (Republika Srpska).
There were unconfirmed reports of injuries as several hundred protesters clashed with police outside the US consulate in Banja Luka.
In the coming weeks, an almost 2,000-strong EU mission will be deployed to help Kosovo develop its police force and judiciary.