The US-led coalition fighting the Islamic State group said one of its bases in northeast Syria came under fire on Wednesday from Iran-backed groups, the latest in a string of attacks.
The development came one day after coalition forces said they had foiled a rocket attack on the same base, located in a part of Syria under the control of Kurdish forces.
"Coalition forces were targeted this morning by eight rounds of indirect fire at Green Village" base, a statement said Wednesday.
"The attack did not cause any casualties, but several rounds impacted inside the coalition base and caused minor damage."
The coalition said it responded by firing six rounds of artillery towards the source of the attack, outside the eastern town of Al-Mayadeen in Deir Ezzor province.
It blamed "Iran-supported malign actors" operating from "civilian infrastructure".
"Our coalition continues to see threats against our forces in Iraq and Syria by militia groups that are backed by Iran," coalition chief Maj. Gen. John W. Brennan, Jr. was quoted as saying.
"The coalition reserves the right to defend itself and partner forces against any threat."
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 Qasem Soleimani and his Iraqi lieutenant in a US drone strike at Baghdad airport.
Earlier on Wednesday, the Britain-based Syrian Observatory for Human Rights said pro-Iran militia fighters fired shells towards a US base in eastern Syria's Al-Omar oil field, causing damage but no casualties.
At least three shells landed 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.
There were no immediate reports of casualties. When asked about the incident, the coalition said it hadn't received reports of new attacks.
The developments in Syria have coincided with similar incidents in neighboring Iraq.
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.