Britain, Germany and the Netherlands have advised their citizens to immediately leave the city of Benghazi in Libya.
The British Foreign Office said on Thursday there is a specific imminent threat to Westerners in the city and advised British nationals there to leave immediately.
Germany and the Netherlands have also issued warnings to their citizens.
The ABC reports British citizens have been told that they should travel through the city as a group as they leave, and not at night.
The United States ambassador to Libya was killed when a consulate in Benghazi was attacked on 11 September.
The Foreign Office has been advising against travel to Benghazi and most parts of Libya since then. Britain has not had a diplomatic presence in Benghazi since the attack.
Benghazi was the stronghold of the National Transitional Council, the group whose revolt eventually ended Col Muammar Gaddafi's hold on power in Libya.
The Foreign Office has made no further comment further on the nature of the threat. However, in an updated travel advice, the Foreign Office said that after the recent French military intervention in Mali, there was the possibility of retaliatory attacks against Western interests in the region.
There is also the threat of kidnapping in Libya.
Last week, in Algeria, a gas plant near In Amenas was taken over, possibly in revenge for events in Mali, where the French military is now operating.
It is thought 37 foreigners died during a four day siege, which ended after Algerian special forces stormed the compound.
The BBC reports that Libya is angry about the warnings, saying it knows of no such threat.