The department also said that due to security concerns and limited staffing it was only able to offer limited emergency services to U.S. citizens there. There was no move yet to evacuate diplomats from the embassy.
The department said the security situation in Libya "remains unpredictable and unstable," with crime levels high in many parts the country. It added that various groups also have called for attacks against U.S. citizens and U.S. interests in Libya.
"U.S. citizens currently in Libya should exercise extreme caution and depart immediately," the department said in a statement.
The department in December warned U.S. citizens against all but essential travel to Tripoli, and against any travel outside the Libyan capital.
Since then a renegade Libyan general has launched a military offensive against Islamists and their allied militias in Libya, which has been plagued by lawlessness since the death of dictator Moammar Gadhafi in 2011.
Also Tuesday, U.S. defense officials said the Navy warship USS Bataan had moved into the Mediterranean Sea and could be used, if needed, for any possible evacuation of American personnel from Libya.
The officials said the Bataan, an amphibious assault ship, was not near Libya and wasn't expected to be positioned off its coast. It more likely would remain in the region, and could move or respond quickly if called.
The officials spoke on condition of anonymity because they were not authorized to discuss the matter by name.