Three suicide drones attacked a US base in eastern Syria on Friday, wounding two Syrian opposition fighters, the US military said.
No Americans were hurt in the attack.
It was not clear who was behind the drone attack, but Iran-backed fighters in the region have carried out such operations in the past. Sleeper cells of ISIS are also active in the area.
Two of the drones were shot down and the third struck the compound at al-Tanf base where the borders of Syria, Iraq and Jordan meet, the US military said.
The military said the drone that struck the building wounded two members of the Syrian Free Army, who received medical treatment.
The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said the attack was likely carried out by Iran-backed fighters who are deployed in different parts of war-torn Syria.
“Attacks of this kind are unacceptable — they place our troops and our partners at risk and jeopardize the fight against ISIS,” US military spokesman Joe Buccino said, using an acronym for ISIS.
There are roughly 900 US troops in Syria, including in the north and the farther south and east of the country.