File photo: US soldiers take part in military drills alongside the Syrian Democratic Forces, east of al-Hasakah on September 7, 2022. AFP
No one claimed responsibility for the attack in east Syria, where it is not uncommon for bases housing U.S. troops to come under rocket fire or mortar attacks.
The Syrian Observatory for Human Rights, a Britain-based war monitor, accused pro-Iranian groups of the attack a day after Iranians marked the third anniversary of the killing of a top general in a US drone strike.
At a commemoration in Tehran on Tuesday, Iran's President Ebrahim Raisi vowed "revenge" for the assassination of Qassem Soleimani, commander of the Quds Force, the foreign operations arm of the Islamic Revolutionary Guard Corps.
The US military's Central Command (CENTCOM), in a statement that avoided accusing any perpetrators, said "two rockets targeted coalition forces at Mission Support Site Conoco" in the eastern Deir Ezzor province "today at approximately 9 am (0600 GMT)".
"The attack resulted in no injuries or damage to the base or coalition property," it added.
Members of the Kurdish-led Syrian Democratic Forces (SDF), the US-backed outfit that defeated the Islamic State (IS) group in Syria, "visited the rocket origin site and found a third unfired rocket", CENTCOM said.
There are roughly 900 U.S. troops in Syria, including in the north and farther south and eas as part of an international coalition fighting IS remnants.
CENTCOM spokesman Joe Buccino said "attacks of this kind place coalition forces and the civilian population at risk and undermine the hard-earned stability and security of Syria and the region".
Syrian bases hosting the US-led coalition have come under sporadic rocket fire blamed on pro-Iranian militias and IS.
Two such attacks in November, targeting a US patrol base in the northeastern Hasakeh province and the coalition's Green Village base near the Iraqi border, caused no casualties or damage, CENTCOM said.
The Syrian Observatory, which relies on a vast network of sources in the war-torn country, said IS was behind a November 26 attack that targeted the base in Hasakeh province's Al-Shaddadi.
The November 17 rocket fire that targeted the Green Village base in Al-Omar originated "from a base of pro-Iranian militias," according to the Syrian Observatory.