William Hague today condemned an attack on the US consulate in Libya that killed the ambassador.
The Foreign Secretary said the death of Chris Stevens and three other staff highlighted the risks faced by diplomats.
President Barack Obama said the assault by an armed mob had been "callous", and praised the victims for "exemplifying America's commitment to freedom, justice and partnership with nations and people around the globe".
The violence in the eastern city of Benghazi yesterday marked a sharp escalation in 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.
Mr Stevens was reportedly killed when a rocket hit his car, although there are conflicting reports about whether he lost his life in the initial incident or as he returned to the scene this morning.
Mr Hague, who is on a visit to Egypt, posted on Twitter: "A sad day for the United States. We feel with them the loss of Ambassador Stevens and his colleagues. Reminds us of dangers diplomats face."
The protests have been sparked by a low-budget film about Islam made in the US and distributed on the internet.
Mr Obama said the violence was "outrageous", and he confirmed that Mr Stevens was among four Americans killed.
"They exemplified America's commitment to freedom, justice and partnership with nations and people around the globe, and stand in stark contrast to those who callously took their lives," he said.
In a statement released before the ambassador's death was confirmed, Mr Hague said there was "no justification" for the attack.
"It is essential that the Libyan authorities take urgent action to improve security, particularly in Benghazi, and identify those responsible for such attacks," he said.
"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."
Downing Street said: "There were no UK staff involved in the incident.
"Clearly, the security of our people is paramount and we strive to ensure that they are safe at all times when they are overseas.
"At the moment, we have no permanent presence in Benghazi."