The US on Thursday branded Serbia’s behaviour as “unacceptable” and condemned the country’s government for what it said was the incitement of violence after scores of protesters broke into the US embassy in Belgrade and set part of it ablaze.

A charred body was found in the embassy, which security personnel later managed to secure. US officials said they believed it was probably that of a protester rather than of an embassy employee.

The violence comes as a contrast to the US’s hopes of a smooth resolution to the long-running dispute over Kosovo, which declared independence from Serbia at the weekend and which the US and leading European Union countries have recognised. It also underlines the obstacles facing Washington and Brussels as they seek to strengthen Belgrade’s ties with the rest of Europe.

Nicholas Burns, US undersecretary of state, who earlier this week spelled out the US’s ambition to “reach out” to Serbia, on Thursday delivered a formal protest to Vojislav Kostunica, Serbia’s prime minister.

“The message was very clear: that the situation was intolerable, that they needed to act immediately to provide the adequate security forces so that our embassy compound and our personnel were not under attack,” said Sean McCormack, State department spokesman.

Mr Burns, who called Mr Kostunica under personal instructions from Condoleezza Rice, US secretary of state, told the Serbian prime minister that no recurrence of Thursday’s events was acceptable and obtained his assurance that no repetition would occur.

“We will hold them to that,” Mr McCormack said. “We have seen a lot of disturbing reports about statements by Serbian government officials, even including a minister, about incitement to violence – that has to cease.”

The United Nations Security Council on Thursday condemned the mob attacks, stressing “the fundamental principle of the inviolability of diplomatic missions.”

Tens of thousands of protesters had converged on Belgrade to insist that Kosovo remained part of Serbia. Government leaders joined with the hardline nationalist Radical party for the huge rally. Hundreds of buses and free transport on the national railways ensured that citizens could come from all over Serbia, with turnout expectations running as high as 1m.

One protester climbed up to the first floor of the building, ripped the US flag off its pole and briefly put up a Serbian flag. Some protesters jumped up and down on the embassy balcony, holding up a Serbian flag as the crowd cheered them on.