Syrian military forces loyal to embattled President Assad ignored warnings about approaching a small US base that houses its Special Operations Forces this week and was hit by a US-led airstrike.

This was the second attack in just the past month against Assad’s forces by US airstrikes in what the Defense Department characterized as self-defense measures.

“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 base, which houses British as well as American Special Operators, is located on a key highway in an area where the borders of Iraq, Jordan, and Syria meet and is considered of vital importance.