It’s not the first time the U.S. military has targeted pro-Assad forces in Syria in self-defense, even though its primary mission in the country is to defeat the Islamic State. | Joseph Eid/Getty Images U.S.-led forces kill an estimated 100 Syrian regime troops

In a major clash the U.S.-led military coalition in Syria has killed more than 100 troops allied with the regime of Bashar Assad — after some 500 soldiers backed by tanks launched what appeared to be a "coordinated attack" on a headquarters of rebel fighters being trained by American advisers, military officials revealed late Wednesday.

The combination of air and artillery strikes came after pro-regime troops attacked the compound of the Syrian Democratic Forces in Khusham, Syria, according to a military spokesman.


One SDF soldier was wounded, said Col. Thomas Veale of the Combined Joint Task Force Operation Inherent Resolve, the command that is directing the battle against the Islamic State terrorist group in Iraq and Syria.

The Syrian Democratic Forces have been a crucial American ally in the fight against ISIS.

"After 20 to 30 artillery and tank rounds landed within 500 meters of the SDF headquarters location, Syrian Democratic Forces supported by the Coalition targeted the aggressors with a combination of air and artillery strikes," the statement said. "Coalition advisers were with the SDF in an advise, assist and accompany capacity, and this action was taken in self-defense."

The pro-Assad forces were supported by artillery, tanks, and rockets in an attack that began late Tuesday and lasted into Wednesday morning local time.

U.S. officials declined to provide details on the military assets that were used to beat back the assault, which nations they were from, or the number of advisers in the area.

Veale said it's likely the pro-regime forces were trying to seize oilfields in Khusham that had been liberated from Islamic State control in September.

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Efforts were undertaken before and after the battle to avoid any confrontation with Russian military forces in the area that are backing the Assad regime in the country's long-running civil war.

"Coalition officials alerted Russian officials of the SDF presence in Khusham via the de-confliction line well in advance of the PRF attack," the statement said. "Coalition officials were in regular communication with Russian counterparts before, during and after the thwarted PRF attack. Russian officials assured Coalition officials they would not engage Coalition forces in the vicinity."

It’s not the first time U.S.-led forces in the country primarily to hunt ISIS have targeted pro-Assad forces in self-defense.

On May 18, they attacked pro-regime forces that continued approaching coalition troops despite warning shots and Russian attempts to halt the troops.

The U.S. Navy also launched a volley of Tomahawk missiles at a Syrian airfield in April after Assad's troops used chemical weapons against civilians.

Last week, Defense Secretary Jim Mattis, responding to reports of renewed gas attacks, warned the Syrian government it would be “ill-advised” to launch more chemical weapons.

“You’ve all seen how we reacted to that,” Mattis said, referring to last year’s cruise missile strikes.

