Foreign Secretary William Hague has condemned an attack on the US consulate in Libya in which the ambassador is believed to have died.
Armed men stormed the base in the eastern city of Benghazi before setting it on fire. The top diplomat was reportedly killed when a rocket hit his car, while several other staff are also thought to have been killed.
The assault on Tuesday came amid protests over an American-produced film said to ridicule Islam and the Prophet Mohammed.
The US embassy in Cairo was also targeted earlier in the day, but no one was injured.
British officials said they were waiting for confirmation of the victims.
Mr Hague said: "There is no justification for such an attack and the appalling death of a US official. My thoughts and condolences are with his family and all his colleagues at the State Department.
"It is essential that the Libyan authorities take urgent action to improve security, particularly in Benghazi, and identify those responsible for such attacks.
"The UK stands ready to assist the Libyan authorities in any way we can and to support their efforts to continue the path towards a stable and secure Libya that fulfils the aspirations of the Libyan people."
US secretary of state Hillary Clinton said one official was killed, and security would be stepped up worldwide.
"We are heartbroken by this terrible loss," she added.