The British Embassy in Belgrade came under attack as protests against Kosovan independence swept the Serbian capital, Foreign Secretary David Miliband said.
Damage to the building was "limited" and embassy staff were safe, Mr Miliband said.
The United States Embassy was torched by Serb rioters and a charred body was found after the attack.
Both the UK and the United States have recognised Kosovo after it declared independence from Serbia earlier this week.
In a statement, Mr Miliband said: "I have been closely watching reports of the violence in Belgrade against international embassies, including the UK embassy.
"I want to underline that we are in close touch with our Ambassador in Belgrade and understand that all our embassy staff are safe, that the premises are currently secure and that damage to the building is limited.
"While the Kosovo issue raises strong feelings in Serbia, no cause can justify such displays of violence.
"We have made clear to the Serbian government that we expect them to fully uphold their obligations to protect our embassy and other diplomatic premises in Belgrade."
Masked attackers broke into the US compound, which was closed at the time, just after 7pm, and tried to throw furniture from an office.
They set fire to the office and flames shot up the side of the building.
The US reacted with unusual sharpness, denouncing Serb authorities for failing to protect the compound from rioters who torched part of its main office building.
"Our embassy was attacked by thugs," White House press secretary Dana Perino said aboard Air Force One as President George Bush and secretary of state Condoleezza Rice returned from a trip to Africa.
"We have made known to the Serbian government our concern and displeasure that their police force did not prevent this incident."
US Embassy spokeswoman Rian Harris said a body was "found at the part of the building set on fire by the protesters".
She said all embassy staff were accounted for. Belgrade's Pink TV said the body appeared to be that of a rioter.
Serbia's president Boris Tadic, on an official visit to Romania, appealed for calm and urged the protesters to stop the attacks and move away from the streets.Reuse content