U.S. air strikes late Wednesday into early Thursday killed more than 100 fighters aligned with Syrian President Bashar al-Assad’s regime, American military officials said.

The U.S. jets and artillery were called in when Syrian “pro-regime forces” moved tanks and artillery and fired mortars about 5 miles east of the Euphrates River in Khusham, Syria.

The river has long been a dividing line between U.S.-backed forces and Russian- or Iranian-backed fighters of the Assad regime.

The build-up of Syrian pro-regime forces took place over the past week and included approximately 500 fighters in a large, formation supported by artillery, tanks, multiple-launch rocket systems and mortars, one official said.

After 20 to 30 artillery and tank rounds landed within 500 yards of U.S. special operations forces advising U.S.-backed forces of the Syrian Democratic Forces (SDF), the U.S. strikes began in “self-defense,” the official said.

Those pro-regime vehicles and personnel who retreated across the river were not targeted, the official added.

Before the U.S. counter-attack began, American military officials called their Russian counterparts on a pre-established hotline to voice concern about the build-up. The Russians assured the Americans the U.S.-backed forces would not be harmed.

One official says Russian contractors may have been involved in the attack.

One U.S.-backed SDF fighter was wounded in the exchange of strikes. No American special ops troops were harmed.

The U.S. military suspects the pro-regime forces were trying to move in on oil fields previously held by the Islamic State group for the past three years, but are now controlled by U.S.-backed fighters, another official said.

The latest U.S. air strikes came more than two weeks after U.S. jets killed as many as 150 ISIS fighters in eastern Syria.

The Associated Press contributed to this report.