British nationals should leave the Libyan city of Benghazi immediately "in response to a specific threat to Westerners", the Foreign Office says.It said the British Embassy in Tripoli had been in contact with British nationals whose details it had.It said it could not comment further on the nature of the threat, but said there was new travel advice for Libya.The Foreign Office has been advising against travel to Benghazi and most parts of Libya since September.BBC world affairs correspondent Caroline Hawley said that after the recent French military intervention in Mali there was the possibility of retaliatory attacks against Western interests.On 11 September, US ambassador Chris Stevens and three other Americans died during an attack on the consulate in Benghazi.The ambassador died of smoke inhalation when he was trapped in the burning building, after armed men had stormed the compound.