US forces in eastern Syria took indirect fire on Saturday but initial reports did not indicate any casualties or damage, a US defense official told Reuters.
The incident was the latest in a series of attacks on US personnel in Iraq and Syria in recent days.
The official, speaking on the condition of anonymity, said the attack occurred in Conoco, Syria.
The Syrian Observatory for Human Rights and Syrian state television confirmed the attack on the Conoco gas plant.
Conoco, in the eastern Deir Ezzor province, is the base of the international anti-ISIS coalition
A mortar shell had landed in a gas field in eastern Syria where the forces are based, said the Observatory on Sunday.
The war monitor said it was likely that militias deployed west of the Euphrates River were behind the incident, which is just the latest in a series of attacks that have targeted the al-Omar oilfield.
Tensions have been high in the region after three US strikes last month. One targeted western Iraq and two targeted Syria. Washington at the time said that pro-Iran groups were using those areas to launch attacks against American forces and manufacture weapons and drones.
Last week, Kurdish Syrian forces said they countered drone attacks near the anti-ISIS coalition base in al-Omar field. Two mortars, fired by unknown sides, landed in the western part of the field. No casualties were reported.
While there have not been immediate claims of responsibility for the attacks, analysts believed they were part of a campaign by Iranian-backed militias.
Iraqi militia groups aligned with Iran vowed to retaliate after US strikes on the Iraqi-Syrian border killed four of their members last month.
The United States has been holding indirect talks with Iran aimed at bringing both nations back into compliance with the 2015 Iran nuclear deal, which was abandoned by the Trump administration. No date has been set for a next round of the talks, which adjourned on June 20.
Seven rounds of talks have so far led to a dead end due to Iran’s unyielding positions and the recent election of hardline Ebrahim Raisi as president will likely only complicate them.
Separately, Deputy to the US Ambassador to the United Nations Jeffrey Prescott said the agreement the US was able to broker with Security Council members to keep vital humanitarian aid flowing into Syria, into Idlib, through the Bab al-Hawa border crossing was a major diplomatic breakthrough.
During a telephone press briefing on Saturday, he hinted at wider American-Russian cooperation over the issue.
The extension “was possible because of the work the United States and Russia were able to do together diplomatically to forge an agreement that meets the dire humanitarian needs of the Syrian people,” he said.
“This humanitarian initiative is a critical starting point, and our aim is to build on it in the months to come,” he stressed.
“We think there’s an opportunity for continued work to meet the urgent humanitarian needs for the people of Syria, and that this is a foundation of a potential humanitarian initiative to better meet those needs. And that’s what the focus of the conversation has been diplomatically between the US and Russia and between the US and other members of the Security Council,” Prescott continued.
“This is a real achievement, demonstrating what patient and quiet diplomacy can do to keep an urgently needed humanitarian – to keep urgent humanitarian assistance flowing through this critical border crossing. So we think there’s a real opportunity to continue to work to expand humanitarian assistance through all modalities. There’s some references to that in the Security Council resolution, and that was the focus of the diplomacy that went into achieving this result,” he remarked
President Joe Biden “had a chance to speak to President Vladimir Putin on Friday and they both welcomed the efforts by our teams to reach this agreement and to be able to provide the continuation of this critical assistance in the year to come,” he went on to say.
“That’s obviously been a priority for the United States, and I think it’s a positive sign and positive signal that we were able to work together.”
“Of course, there’s a lot of additional work that we need to do. We’ve urged the Security Council – and the United States will continue to push – to expand humanitarian access so that everyone in need can get the assistance that they desperately need,” Prescott added. “The United States supports expanding the number of crossings.”
“Now, obviously, there are a whole host of other issues where we have disagreements with Russia, and the President and Putin were discussing some of those issues in their call yesterday, as the readout makes clear. But this is a positive outcome; it’s a good example of what diplomatic efforts between the United States and Russia can achieve.”