American airstrikes targeted a group of Syrian militia vehicles after they ignored U.S. warnings and violated a “de-confliction zone” on Thursday.

CNN reports that the strike occurred near a base American forces use to train allied Syrian opposition fighters.

A convoy of twenty vehicles approached the town of An Tanf on Wednesday night, evidently searching for opposition fighters and raising concerns among the U.S. coalition. A total of thirteen vehicles penetrated the de-confliction zone, but the U.S. did not take action until five of them came with 29 kilometers of the base.

When five Syrian military vehicles persisted in approaching the base, the U.S. conducted a “show of force” with two warplanes to persuade them to halt. When the Syrians insisted on proceeding into the area, the U.S. planes were cleared to fire.

Although one of CNN’s sources said it remained unknown whether the American planes only fired warning shots, a second U.S. defense official said an airstrike “did hit the convoy after the vehicles continued toward the base.”

According to ABC News, U.S. officials said several of the Syrian vehicles were destroyed in the strike. Voice of America News reports the decision to launch the airstrike was made by a commander on the ground, and does not reflect a broader change in U.S. policy.

According to these sources, militia forces loyal to the regime of Bashar Assad crewed the vehicles, not regular Syrian army troops. Military Times quotes officials describing the targeted militia as “pro-regime” but “not directly associated with the Syrian government.”

The Associated Press quotes officials who said the targeted vehicles included “a tank and a bulldozer,” which pro-regime militia were apparently using to set up fighting positions inside the protected area.

Ominously, the Syrian regime has its own base fairly close to the one American forces are using to train opposition fighters, and the Syrian base is said to be ready to support “about a battalion’s worth of troops.”