A US official in Washington says a charred body has been found in the US embassy in Belgrade, which was attacked and set on fire this morning by Serb rioters protesting at Kosovo's declaration of independence.

US State Department spokesman Sean McCormack said that all American personnel at the embassy had been accounted for and there was nothing to suggest the body was an embassy employee.

Mr McCormack could not say that all embassy personnel who were not US citizens were accounted for.

William Wanland, a spokesman for the US Embassy in Belgrade, told CNN the body appeared to have been a protester.

"It appeared to have been a protester who was caught in the fire," he said, adding the death had not been caused by any interaction with US security forces at the embassy.

Mr Wanland said about 70 American diplomats served in the Belgrade embassy.

The US ambassador to the United Nations says he's "outraged" at the actions of the rioters.

The storming of the US embassy came during a state-backed rally to protest at Kosovo's secession attended by around 200,000 people, which was otherwise peaceful.

Some rooms at the embassy were set on fire.

Police were not protecting the building, which had been closed and boarded up after rioters stoned it earlier in the week.

Black smoke billowed out of the embassy. Papers and chairs were thrown out of the windows, with doors wedged in the window frames and burning.

One protester climbed up to the first floor of the building, ripped the Stars and Stripes off its pole and briefly put up a Serbian flag in its place.

Some protesters jumped up and down on the embassy balcony, holding up a Serbian flag as the crowd below of about 1,000 people cheered them on, shouting "Serbia, Serbia".

The rioters were mainly young men, some of whom wore balaclavas and scarves to hide their faces.

They had attacked the building with sticks and metal bars and destroyed two guard boxes outside.

Protesters ripped some metal grilles from the embassy windows and also tore a handrail off the building's entrance and used it as a battering ram against the main door.

- ABC/AFP/Reuters