Residents walk through damaged streets at the mountain resort town of Zabadani in the Damascus countryside, in Syria, May 18. (Hassan Ammar/AP)

American aircraft struck a convoy of troops loyal to Syrian President Bashar al-Assad on Thursday, in a rare direct assault by the United States on forces affiliated with the Syrian government.

Assad’s government had no immediate response to the incident, which occurred in far southern Syria near an area where American Special Operations forces are working with local fighters.

A defense official, speaking on condition of anonymity to discuss details of an overseas operation, said that U.S. aircraft struck a convoy of several dozen vehicles, including a small number of tanks, that were advancing toward a U.S.-occupied garrison in Tanf, along the Syria-Iraq border.

“This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south towards [Tanf] were unsuccessful,” a Pentagon statement said.

U.S. officials said the vehicles were slowly advancing within a 30-mile ground and air exclusion zone that the United States has established around Tanf.

U.S. forces pictured in in central Syria. (Delil Souleiman/AFP/Getty Images)

Before the attack, the aircraft conducted a show of force by flying by the convoy and fired warning shots, the official said. The Syrian fighters “did not choose to turn away,” the official said.

The strike destroyed four to five vehicles, including several construction vehicles and at least one tank, and appeared to have killed those inside, the official said. U.S. officials said they were unsure whether the forces in the convoy were Syrian army troops or members of militia groups who fight on the government’s behalf.

Salim Zahran, an analyst close to the Syrian government, said that U.S. planes targeted a Syrian army battalion in the Shahma area, 40 miles from the Tanf border area. He said the strike, which destroyed tanks and four-wheel-drive vehicles, killed eight soldiers from the 13th Battalion.

Zahran suggested reports that foreign militiamen were among the dead were incorrect.

The attack takes place as pro-government forces advance on U.S.-backed rebels in southern Syria in an apparent attempt to prevent them from expanding their foothold there.

It underscores the risks the United States faces as it continues its campaign against the Islamic State in Syria, where the government has been bolstered by Russian and Iranian combat power.

The Trump administration has embraced escalation of U.S. military action in Syria, as it did in April when it launched missile strikes on a Syrian air base in response to a chemical attack on civilians.

While the Pentagon hopes that allied Syrian forces will launch an operation against the city of Raqqa, the Islamic State’s de facto capital, in coming weeks, Thursday’s attack, and the April air base assault, mark rare deviations from U.S. efforts to avoid plunging directly into the civil war that has consumed Syria since 2011.

Asked about the Tanf episode Thursday, Defense Secretary Jim Mattis said the strike would not draw the United States further into the war.

“We are not increasing our role in the Syrian civil war, but we will defend our troops,” he said during a visit of Sweden’s defense minister to the Pentagon. “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.”

U.S. officials said they had used a communications channel that seeks to avoid in-air mishaps between U.S. and Russian aircraft to ask Russia, one of Assad’s chief backers, to notify the members of the convoy to retreat. But they said it was unclear whether that message had been delivered.

Louisa Loveluck and Suzan Haidamous in Beirut and Kareem Fahim in Istanbul contributed to this report.