Pro-Iran militia fighters fired shells towards a US base in eastern Syria's Al-Omar oil field on Wednesday, causing damage but no casualties, the Syrian Observatory for Human Rights said.
The development came one day after the US-led coalition fighting the Islamic State in Syria said they had foiled a rocket attack on a US base called "Green Village" in the eastern Deir Ezzor region.
Wednesday's attack, also in Deir Ezzor, saw at least three shells land in Al-Omar, according to the Observatory, which relies on a network of sources inside Syria.
One fell on a helicopter landing pad, while the two others hit an open area, the Observatory said.
The war monitor blamed the attack on pro-Iran militia groups that are active in the Deir Ezzor region which borders Iraq.
It said coalition forces retaliated by striking the source of the artillery fire outside the eastern town of Al-Mayadeen which is a hub for Tehran and its Syria proxies.
There were no immediate reports of casualties.
The attacks targeting US installations came as Tehran and its allies across the Middle East held emotional commemorations marking the second anniversary on Monday of the assassination of Iranian commander General Qassem Soleimani and his Iraqi lieutenant in a US drone strike at Baghdad airport.
On Tuesday, US-led coalition forces shot down two armed drones targeting an air base in western Iraq, according to a coalition official.
On Monday, the coalition shot down two armed drones targeting its compound at Baghdad airport.