The military stated that soldiers shot an assailant, described only as a Syrian national. The gunman was wounded and transported to a hospital, AP reported.
The US embassy confirmed that the assailant opened fire with small arms near the embassy entrance. No members of the diplomatic corps were injured, the embassy stated.
American diplomats are in close contact with Lebanese law enforcement officials.
The attacker shot a guard at the US embassy, and Lebanese forces returned fire, injuring the gunman, The National reported, citing a diplomatic source.
Tight security measures have been implemented near the embassy in the Beirut suburb of Aaoukar following the incident.
Earlier, the Lebanese NNA news agency reported that shooting was heard in the area of the US embassy.
"The US embassy in Lebanon was shot at by a person with Syrian citizenship. Military in the area responded to the gunfire, and the shooter was injured. He was detained and sent to hospital for treatment. Work is underway to establish the circumstances of the incident," the armed forces said in a statement.
The shooter's motives remain unclear.
Local media reported a gunfight lasting nearly half an hour near the US diplomatic mission in the suburb of Aukar, north of Beirut.
A Lebanese security official told The Associated Press that there were four assailants, including one who drove the gunmen to the site and three who opened fire.
SD/PR