Lebanese authorities have arrested 20 people after a shooting near the US embassy in Beirut said to be in solidarity with Palestinians in Gaza, a judicial source told AFP Saturday.

A Syrian man was arrested Wednesday after the shooting at the embassy, and a judicial official said at the time that the assailant carried out the attack "in support of Gaza".

The US embassy said on X that "small arms fire was reported in the vicinity of the entrance" and that "thanks to the quick reaction" of the Lebanese army, security forces "and our embassy security team, our facility and our team are safe".

The judicial source on Saturday said "the number of people arrested over the attack on the American embassy has risen to 20, including the Syrian assailant Qais Farraj, who is receiving care at the military hospital in Beirut".

"Among those arrested are his father, his brother, clerics who gave him religious lessons and those with whom he was in continuous contact," added the source, who supervises preliminary investigations by intelligence services.

The source added that more people could be arrested or released based on the outcome of the investigation, which is seeking to determine whether any of the suspects had links to an Islamic State group cell or any other militant groups.

But the source said existing evidence suggested it was unlikely the attack was part of an organised operation.

A security official told AFP at the time of the attack that the gunman acted "alone", adding that a Lebanese national working for the embassy suffered light injuries to his eye.

US State Department spokesman Matthew Miller later said Washington was aware the person arrested appeared to be wearing "ISIS insignia", referring to the Islamic State group.

The United States, he said, was "conducting a full investigation with the Lebanese authorities into the actual motivations".