Ambassador Christopher Stevens and other members of the U.S. diplomatic corps died when militants attacked the U.S. Consulate in Benghazi.
The British government warned that violent clashes are possible, especially at night, in parts of the country previously described as peaceful.
"We advise against all but essential travel to Zuwara, Az Zawiya, Tripoli, al-Khums, Zlitan and Misurata, and the coastal towns from Ras Lanuf to the Egyptian border, with the exception of Benghazi," the warning stated. "We advise against all travel to all other areas of Libya, including Benghazi."
Condolences poured in from world leaders after Stevens' death was confirmed Wednesday. U.S. President Barack Obama vowed to bring justice to those responsible for the attacks. NATO Secretary-General Anders Fogh Rasmussen said the level of violence "can never be justified."
"I welcome the Libyan president's condemnation and condolences and his pledge of his government's full cooperation," said Rasmussen. "It is important that the new Libya continues to move towards a peaceful, secure and democratic future."
The International Committee of the Red Cross pulled some of its staff from Benghazi and Misurata early this year after workers were attacked.
Similar attacks in Egypt broke out in protest of a controversial U.S.-made film that was seen as insulting to the Prophet Muhammad.
The Taliban in Afghanistan issued a statement calling for revenge against the U.S. government "by dealing a heavy blow to its invading troops on the battlefield."
Stevens apparently died from smoke inhalation. He was 52.