This file handout photo shows a pair of US Air Force F-15E Strike Eagles flying over Iraq. File/AFP

Colin Kahl, the undersecretary of defense for policy, told reporters that the US airstrikes overnight on facilities used by militias backed by Iran’s paramilitary Revolutionary Guard demonstrated that "the United States will not hesitate to defend itself against Iranian and Iran-backed aggression when it occurs.”

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He said the US decision to launch the strikes was based on both the nature of the militia attacks on Aug. 15 at the Al Tanf Garrison, where US troops are based in the south, and the fact that, based on recovered drone parts, "we believe we have Iran dead to rights on attribution.”

Hours after the US strikes, two US military locations in northeastern Syria near large oil and gas fields were hit with rocket fire. According to US Central Command, the rockets hit near Green Village and the Conoco gas field in Deir El-Zour. One US service member was treated for a minor injury and has been returned to duty, and two others were under evaluation for minor injuries. CENTCOM said US forces responded by destroying three vehicles and equipment used to launch some of the rockets, killing "two or three suspected Iran-backed militants.”

A US AH-64 Apache attack helicopter flies over Baghouz, in the eastern Syrian Deir Ezzor province. File/AFP

The opposition war monitor the Syrian Observatory for Human Rights and Deir Ezzor 24 said the US airstrikes targeted the Ayash Camp run by the Fatimiyoun group made up of Shiite fighters from Afghanistan. The war monitor reported that at least six Syrian and foreign militants were killed in the airstrikes, while Deir Ezzor 24 reported 10 deaths.

Deir el-Zour is a strategic province that borders Iraq and contains oil fields. Iran-backed militia groups and Syrian forces control the area and have often been the target of Israeli war planes in previous strikes.

In Iran, Foreign Ministry spokesman Nasser Kanaani issued a statement condemning the American strike "against the people and infrastructure of Syria.” He denied Iran had any link to those targeted.

Iran routinely denies arming militia groups that target US forces in the region, despite weaponry linking back to them.

Kahl said the US strikes underscore that while the US continues to pursue negotiations with Iran to resume its compliance with the 2015 nuclear deal, those talks are not connected at all to America's willingness to take against when attacked.

"The threats that they engage in against our people in the region or elsewhere, are not linked to wherever we end up on the nuclear deal,” said Kahl. "It actually has nothing to do with our willingness and resolve to defend ourselves. And I think the strike last night was a pretty clear communication to the Iranians that these things are are all on different tracks.”

Associated Press