From the U.S. State Department:

The Department of State warns U.S. citizens against all travel to Libya. On September 12, 2012 the Department of State ordered the departure of all non-emergency U.S. government personnel from Libya, following the attack on the U.S. Diplomatic mission in Benghazi. The political violence has increased in both Benghazi and Tripoli. The airports in Benghazi and Tripoli are open and U.S. citizens are encouraged to depart by commercial air. ...

U.S. citizens traveling to, or remaining in, Libya should use extreme caution and limit nonessential travel within the country, make their own contingency emergency plans, enroll their presence in Libya through the Smart Traveler Enrollment Program (STEP), and provide their current contact information and next-of-kin or emergency contact information.

The Embassy’s website, includes consular information and the most recent messages for U.S. citizens in Libya. U.S. citizens in need of emergency assistance should call 091-379-4560 within Libya or 218-91-379-4560 if dialing from outside of Libya. ...

U.S. State Department Warning