The army said the attacker, a Syrian national, was taken to hospital for treatment and soldiers were searching through the area.
The U.S. Embassy said small arms fire was reported near its entrance in the morning. The facility and staff were safe, it added.
A security source told Reuters a member of the embassy’s security team was wounded in the attack and that Lebanese soldiers wounded an attacker in the stomach.
The embassy lies north of Beirut in a highly secured zone with multiple checkpoints along the route to the entrance. It moved there from Beirut following a suicide attack in 1983 which killed more than 60 people.
In September, shots were fired near the embassy with no injuries reported.
In October, scores of protesters gathered outside the embassy to demonstrate in the early days of the Gaza war, and Lebanese security forces used tear gas and water cannon to repel them.
Lebanon has been the scene of conflict between Iran-backed Hezbollah and Israel since October in parallel to the Gaza war. Tens of thousands of people on both sides of the Israel-Lebanon border have been uprooted amid fears that the war will deepen.
The United States has been making diplomatic efforts to ease violence along the border.
