EU officials have called on Serbia to do more to protect foreign embassies targeted in protests against Kosovo's secession.
EU foreign policy chief Javier Solana told reporters in Slovenia that the violence could harm progress on a preliminary deal on ties with the union.
Enlargement Commissioner Olli Rehn said the European Union respected the democratic right of the Serbian people to voice their opinion on developments in Kosovo, 'but the use of violence for expressing one's opinion is unacceptable,' he said.
Russia's envoy to NATO warned that formal backing for Kosovo's independence by the EU or NATO would give Russia the right to use brute force in future.
Russia opposed Kosovo's independence declaration, reflecting Moscow's close ties with the leadership in Belgrade.
In Croatia, police arrested 44 people who burned the Serbian flag in Zagreb's main square after Serb protesters attacked the Croatian embassy in Belgrade.
A Croatian foreign ministry spokesman said the two incidents did not bode well for bilateral relations, which were established after the war ended in 1995.
Serb rioters set fire to the US embassy in Belgrade last night, prompting angry US and UN protests, although Russia argued the violence was understandable.
Nearly 100 people were injured in the unrest that followed a largely peaceful rally of 150,000 demonstrators against Kosovo's declaration of independence from Serbia on Sunday.
A charred body later recovered from the US embassy was not identified, but an embassy spokeswoman said the victim was not a staff member.
The US State Department lodged a stern complaint, with Under Secretary of State Nicholas Burns calling Serbian Prime Minister Vojislav Kostunica and Foreign Minister Vuk Jeremic to formally protest.
Although riot police eventually arrived to disperse the rioters, there was particular criticism of the apparent lack of Serbian security during the rally.
A spokeswoman for US President George W Bush said he had been briefed about the incident.
Other embassies of countries that have recognised Kosovo's break from Serbia were also targeted in the rioting.
So far, 23 of the 27 EU member states have backed Kosovo's independence.
As well as the arson attack on the US embassy, a German embassy guard house was torched and a car set ablaze outside the Canadian diplomatic mission. Other embassies targeted included those of Bosnia, Croatia and Turkey.
The rioters later moved to other parts of the city, setting light to a McDonald's restaurant, ransacking a Levi's clothing shop and using uprooted street signs and even trees to fight police.
