The US-led coalition said it repelled an unprovoked attack near the Euphrates River by hundreds of troops aligned with Syrian President Bashar Assad, who were backed by artillery, tanks, multiple-launch rocket systems and mortars.
Mattis described the attack on the US-backed fighters, who were accompanied by US special operations forces, as “perplexing.” But he described the retaliatory US-led coalition strikes as defensive and limited in nature.
Asked whether the US military was stumbling into Syria’s broader conflict, Mattis said: “No. This is self-defense.”
“If we were getting involved in a broader conflict, then it would have had an initiative on our part,” Mattis told reporters at the Pentagon.
“For some reason, pro-regime forces — and again, I cannot give you any explanation of why they would do this — moved against SDF positions” where US special forces were also on the ground, Mattis said.
The pro-Damascus forces “began shelling it with artillery,” he added. “They were moving with tanks, obviously in the same direction as they were firing.
“At the end of our effort to defend ourselves, their artillery was knocked out, two of their tanks were knocked out, they had casualties.”
Mattis did not offer a specific death toll. The Pentagon would not confirm the figure given by the US official late Wednesday.
The US-led side quickly contacted their Russian counterparts to attempt to ensure the situation did not escalate, Mattis said, adding: “The Russians, at that time, were telling us they did not have forces there.”
The fighting took place less than 10 km east of the so-called “deconfliction zone” set up by the US and Russia along the Euphrates River, with Russian forces to the west and US forces to the east.
Moscow and Washington coordinate their air and ground operations against Daesh in this zone.
Mattis said he could not identify the attackers.
“We know they were pro-regime forces but... Iranian, Assad, Russians, mercenaries... I can’t tell you,” he said.
He explained that the US-led coalition had picked up on the movements of pro-regime forces in the area a few days before, but they were not immediately hostile.
Pentagon spokeswoman Dana White told a news briefing that Washington was not “looking for a conflict with the regime.”
The incident underscored the potential for further conflict in Syria’s oil-rich east, where the US-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias holds swathes of land after its offensive against Islamic State.
Assad, who is supported by Russia and by Shiite militias backed by Iran, has said he wants to take back every inch of Syria.
The pro-regime forces were “likely seeking to seize oilfields in Khusham” east of the Euphrates in Deir Ezzor province, said a US official on condition of anonymity.
US Sen. Tim Kaine, who sits on Senate foreign relations and military oversight committees, said the episode raised serious concerns about the open-ended US military presence in Syria.
“I am gravely concerned that the Trump administration is purposefully stumbling into a broader conflict, without a vote of Congress or clear objectives,” Kaine said.
No US or US-backed forces died but the US official who spoke anonymously estimated that more than 100 pro-Syrian regime forces were killed in the counter-attack.
The US-led coalition was set up in 2014 to battle Daesh in both Syria and Iraq, who were largely defeated last year. Some 2,000 US forces remain on the ground in Syria, allied to the Kurdish-led SDF alliance, which holds the largest swathe of territory still outside the control of the government.