The US military said attacks were launched on two facilities on Wednesday evening, with several rockets landing inside the perimeter of Mission Support Site Conoco in northeast Syria, quickly followed by another near Mission Support Site Green Village, both run by the Syrian Democratic Forces, a US-backed Kurdish-led group.

In response, raids by a US helicopter killed “two to three … suspected Iran-backed militants”. The statement said two US military personnel were treated following the attacks, and two others were under evaluation.

The attacks come a day after the US military said it had carried out raids in Deir Az Zor, a strategic oil-rich province that borders Iraq, on groups linked to Iran’s Islamic Revolutionary Guard Corps (IRGC).

US Central Command said the raids were ordered by President Joe Biden and were a direct response to an August 15 attack on the al-Tanf Garrison, where US troops are based.

Colin Kahl, the US undersecretary of defence for policy, told reporters that those raids demonstrated that “the United States will not hesitate to defend itself against Iranian and Iran-backed aggression when it occurs”.

The attacks involved eight US fighter jets, four F-16s and four F-15Es, and hit nine targets in Syria, the military said, including an ammunition depot and logistic supply bunkers.

Iran has denied any links to the fighters, with its foreign ministry on Wednesday calling the US raids a “violation by the US army of the people and infrastructures of Syria”, which it said also amounted to a violation of the country’s territorial integrity and independence.

The Syrian Observatory for Human Rights said the US air attacks targeted the Ayash Camp run by the Fatimiyoun group, made up of Shia fighters from Afghanistan. The opposition war monitor reported that at least six Syrian and foreign fighters were killed in the air attacks.

The latest escalation comes as parties to the 2015 Iran nuclear deal build momentum towards bringing back the landmark agreement, with Washington on Wednesday responding to Tehran’s suggestions for reviving the accord, from which former US president Donald Trump withdrew in 2018.

Kahl told reporters that the state of nuclear deal negotiations would not have an effect on the US’s “willingness and resolve to defend ourselves”.

“I think the strike last night was a pretty clear communication to the Iranians that these things are all on different tracks,” he said.

US forces first deployed to Syria during a campaign against ISIL (ISIS) launched by former US President Barack Obama, partnering with the SDF.

There are about 900 US troops still in the country, with Brett McGurk, the White House coordinator for the Middle East and North Africa, saying last year the troops had four main objectives in Syria: to reduce violence, maintain military pressure on ISIL, address Syria’s humanitarian crisis, and to support Israel.

The Syrian government has constantly expressed its opposition to the US role in Syria, and demanded that US forces withdraw.