Javier Solana, the EU's foreign policy chief, warned continued violence would stall talks on Serbia's inclusion into the union.
Call for calm
Solana said on Friday: "These acts of violence lead nowhere and they cannot help anybody."
He told reporters that negotiations on an agreement designed to prepare Serbia for eventual EU membership would have to wait until things "calm down."
"The embassies have to be protected, that's the obligation of a country."
Solana is in Brdo Pri Kranju, Slovenia, for a meeting of EU defence ministers.
On Thursday, the UN Security Council condemned "in the strongest terms the mob attacks against embassies in Belgrade" and said that it welcomed the steps taken by the Serbian authorities to restore order and protect diplomatic property and personnel.
Zalmay Khalilzad, the US ambassador to the United Nations, had said he was "outraged" by the attack.
Rioters broke into the US mission on Thursday night and set fire to offices and to police guardhouses on the pavement in front of the building.
The nearby Croatian embassy was also attacked, and a residential building next door was damaged by flames.
Firemen put out the fires and found a charred body inside the US mission's consular section.
Media reports said the body may have been that of one of the rioters who set had fire to the office.
After breaking up the protests, riot police fought running battles in the capital's central area, while dozens of looters broke into shops following a state-sponsored demonstration against Kosovo's independence in which nearly 200,000 people took part.
It was the worst rioting in the Serbian capital since 2000, when pro-democracy protesters confronted the security forces of Slobodan Milosevic, the Communist-era leader, in an uprising that led to his removal.
Belgrade's emergency centre said 96 people, a third of them police, had been treated for light injuries in the night. There were more than 100 arrests, police said.
On Friday, a McDonald's in the city centre was still smouldering. Shops were putting up plastic sheeting and panels to cover their smashed windows.
Several sports stores had been cleaned out by.
More than a dozen nations, including the US, Britain, France and Germany, have recognised Kosovo's declaration of independence, however.
But the declaration has been rejected by Serbia's government and ethnic Serbians who populate northern Kosovo.
Russia, China and a number of other nations have also condemned the move, saying it sets a precedent that separatist groups around the world will seek to emulate.