The State Department said it was urging U.S. citizens to avoid traveling to Lebanon and said all U.S. employees in the country should keep a low profile.
"The potential in Lebanon for a spontaneous upsurge in violence is real," the warning read. "Lebanese government authorities are not able to guarantee protection for citizens or visitors to the country should violence erupt suddenly."
The warning said that anti-Western groups like Hezbollah remain a persistent threat and sporadic violence in Hezbollah strongholds in the south of the country makes travel risky.
Last month, seven Estonian bicyclists were kidnapped from the area in what the State Department says was a planned attack. Their whereabouts remain unknown.
Meanwhile, Beirut warned that the findings from a tribunal probing the 2005 assassination of former Prime Minister Rafik Hariri could spark civil unrest. Indictments are expected later this year.
Ongoing conflict between Hezbollah and Israel suggests to the State Department that rocket attacks in the region can erupt with no warning.
The State Department said the threat to U.S. government workers is serious enough to encourage vigilance. Unofficial travel to the country is discouraged.