Two American military bases in eastern Syria were fired at on Friday night, a day after the US retaliated against earlier attacks with air strikes on Iran-backed armed groups in the region.
Three rockets were fired at a US base in eastern Syria, near Al Omar oilfield, the Syrian Observatory for Human Rights said.
The rockets were launched from areas controlled by pro-Iran militias in the town of Mayadin in Deir Ezzor province, the UK-based watchdog said.
The salvo hit houses at the base and a meeting hall of the international anti-ISIS coalition, causing material damage, with no reports of casualties.
The Observatory said Iran-backed militants also attacked another base hosting American forces in Syria's Conoco gasfield.
In response, the US bombed positions of the militant groups in Deir Ezzor. Explosions were heard in the city, the Observatory said.
On Saturday, an Iranian security spokesman said strikes on Iran-linked bases in Syria would draw a quick response.
“Any pretext to attack bases created at the request of the Syrian government to deal with terrorism and Islamic State elements in this country will be met with an immediate counter-response,” Keyvan Khosravi, spokesman for Iran's Supreme National Security Council, was quoted as saying by the semi-official news agency Nournews.
Iran said its forces and allied fighters are in Syria at the request of Damascus, and regards US forces as occupiers.
The US had earlier conducted “precision air strikes” on Iran-linked groups in the same province in response to a drone attack that killed an American contractor at a coalition base near Hasakah in north-east Syria on Thursday.
Five US servicemen and another contractor were wounded in the attack
The Observatory said on Saturday that the death toll from the US strikes had risen to 19 people, including 11 Syrians.
Hours after the US strikes, 10 rockets were fired at American and coalition forces at the Green Village base in north-east Syria, the US Central Command (Centcom) said.
There were no injuries or damage to infrastructure at the base, but a rocket hit a home about five kilometres away, causing minor injuries to two women and two children, Centcom added.
The latest attacks on American targets came as pro-Iranian forces in Syria said that they had a “long arm” to respond to further US strikes on their positions.
An online statement, signed by the Iranian Advisory Committee in Syria, said US strikes had left several fighters dead and wounded, without specifying their nationality.
“We have the capability to respond if our centres and forces in Syria are targeted,” the statement said.
The US has about 900 troops in bases and posts across north-eastern Syria as part of the international coalition fighting remnants of ISIS.
American troops also support the Syrian Democratic Forces (SDF), the Kurds' de facto army in the area, which led the battle that dislodged ISIS from the last scraps of their Syrian territory in 2019.
US personnel have frequently been targeted in attacks by militia groups.
“My heart and deepest condolences to the family of the American we lost and wish the speedy recovery for those who are wounded,” US President Joe Biden said on Friday.
“Make no mistake, the United States does not, does not, I emphasise, seek conflict with Iran. But be prepared for us to act forcefully to protect our people.”