A U.S.-backed anti-government Syrian fighter from Maghaweer al-Thawra stands with a heavy machine gun on his armored vehicle at the Syrian-Iraqi crossing border point of Tanf, south Syria. (Hammurabi’s Justice News via Associated Press)

U.S.-led aircraft struck a column of pro-Syrian-government forces advancing toward a small U.S. outpost in southeastern Syria on Tuesday, the Pentagon said in a statement.

The strike marks a second time in less than a month that U.S. aircraft have targeted troops aligned with Syrian President Bashar al-Assad. Both times, the Pentagon said, were in self-defense.

“Despite previous warnings, pro-regime forces entered the agreed-upon de-confliction zone with a tank, artillery, anti-aircraft weapons, armed technical vehicles and more than 60 soldiers posing a threat to Coalition and partner forces based at the At Tanf Garrison,” the U.S.-led coalition said in a statement.

The statement said the U.S.-led coalition communicated with the advancing forces through an existing hotline set up to ensure that Russian and U.S. aircraft can operate safely in Syrian airspace. Despite the warnings, the Syrian forces continued to advance, prompting the U.S.-led coalition to destroy two artillery pieces and an anti-aircraft weapon, according to the statement. A tank was also damaged.

It is unclear how many troops were killed or wounded in the bombing run and what legal rationale might cover the U.S.-led forces continued strikes on Syrian government forces operating within their borders.

On May 18, U.S. forces struck a convoy of pro-Syrian-government forces that were driving in the direction of the U.S.-base at Tanf. After warnings, both through the hotline and overhead passes, U.S. aircraft struck a number of their vehicles. That convoy, roughly 18 miles from the U.S. outpost, came to halt. In response, the U.S.-led coalition added additional “combat power” to help defend the Tanf base and dropped more than 90,000 leaflets telling the Syrian forces to leave.

“We are not increasing our role in the Syrian civil war, but we will defend our troops,” Defense Secretary Jim Mattis told reporters after the May strike. “And that is a coalition element made up of more than just U.S. troops, and so we will defend ourselves [if] people take aggressive steps against us.”

The Tanf outpost, located near the Syrian, Iraq and Jordanian border, has become a flashpoint in the six-year-old Syrian conflict in recent weeks. Iranian-backed Shiite militias loyal to Assad, along with Syrian government forces, are making a concerted push to recapture territory from the Islamic State in the area in a likely attempt to secure the border and the desert highway that runs into Iraq.

Meanwhile, the U.S.-led coalition and a contingent of U.S. and British Special Operations troops have been training a detachment of Syrian fighters there to ultimately lead an offensive into the Euphrates River valley.