Pro-Iranian militias on Monday evening fired several shells at a US base in eastern Syria's Al-Omar oil field, causing damage but no casualties, the Syrian Observatory for Human Rights said, according to AFP.
The development came after the US launched air strikes overnight Sunday against three targets it said were used by pro-Iran groups in eastern Syria and western Iraq.
In retaliation for Monday evening’s shelling, "heavy artillery has been fired by the (US-led) international coalition" on the town of Al-Mayadeen, controlled by Iranian militias, the Observatory said.
Syrian state news agency SANA said that "missiles... targeted a military base of the US occupation forces in the Al-Omar oil field," without citing who was responsible.
The US military said that Sunday night’s air strikes were in response to drone attacks by the Iranian-backed militia against US personnel and facilities in Iraq.
The strikes targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq, the Pentagon said.
“As demonstrated by this evening’s strikes, President Biden has been clear that he will act to protect US personnel,” it added.
Rocket attacks have regularly targeted Iraqi bases as well as the fortified Green Zone in Baghdad, where the US embassy is located, since the US elimination of top Iranian General Qassem Soleimani last January.
Sunday’s air strikes are not the first time that the Biden administration has retaliated for attacks on US targets in Iraq.
In February, the US bombed a border depot in Syria which the Pentagon said was used by Iran-backed Iraqi armed militia that have been tied to the rocket attacks. The strikes followed a rocket attack targeting an airbase in Iraq's Kurdistan region, which killed a foreign civilian contractor and wounded five others, among them a US soldier.
US President Joe Biden described the February attack as a "warning" to Iran.