A group of U.S. troops stationed at a coalition base in southern Syria were instrumental in fending off a vicious assault by ISIS militants on Saturday, April 8, CNN reports.

ISIS initiated the complex attack on the At Tanf Garrison with a vehicle-borne IED, U.S. Central Command announced in a news releaseSunday. Between 20 and 30 militants, several outfitted with suicide vests, followed the initial attack with a ground assault.

“Coalition and partnered forces engaged and defended the ISIS attack with direct fire before destroying enemy assault vehicles and the remaining fighters with multiple coalition airstrikes,” Centcom said.

It is unclear how many Americans were involved in the battle or how long it lasted. Three coalition soldiers were killed in the attack, according to Centcom. Their identities and nationalities have not yet been released. 

The An Tanf Garrison is located in the Hamad Desert 10 miles north of the Jordanian border and houses around 200 U.S.-supported rebels. The coalition refers to those rebels, who were targeted by Russian airstrikes last year, as “Vetted Syrian Opposition forces,” or VSO. It’s worth noting that the VSO forces are not the same group partnered with the coalition in northern Syria, according to CNN.

“[VSO forces] have been instrumental in countering the ISIS threat in southern Syria and maintaining security along the Syria-Jordan border,” the coalition said.

The attacks come after the U.S. launched 60 Tomahawk cruise missiles at a Syrian regime air base south-east of Homs in retaliation for a chemical weapons attack that killed dozens of civilians.

The strikes were the first direct assault on the government of President Bashar al-Assad, and seemed to signal the Trump administration’s willingness to intensify U.S. military involvement in the six-year-old Syrian Civil War.

Hundreds more American troops have been deployed to Syria since President Donald Trump’s took office. The primary mission of U.S. ground troops deployed to Syria is to assist a patchwork of local rebel groups in the fight against ISIS as the coalition advances toward the stronghold of Raqqa.

The U.S.-led coalition has so far maintained a relatively low casualty rate in Syria.

In November 2016, Senior Chief Petty Officer Scott C. Dayton, a U.S. Navy bomb disposal technician, died after being struck by an IED near Raqqa. Dayton’s death marked the first time an American service member was killed in Syria since U.S. troops began deploying there in 2015. 

The assault on the An Tanf Garrison appears to be the first time a coalition base in Syria has come under direct attack from enemy forces.