Acting to protect American forces based in the region, U.S. warplanes on Thursday struck forces believed to be loyal to Syrian President Bashad Assad, officials said, in what was viewed as another sign of escalating U.S. engagement in the civil war ravaging Syria.
The airstrikes mark the first time the U.S. military has deliberately hit pro-government forces in response to a perceived threat against American troops. The number of U.S. forces in Syria has steadily increased in recent months.
The U.S.-led coalition in Syria said in a statement Thursday evening that it had struck "pro-regime forces that were advancing well inside an established de-confliction zone" near the city of Tanf on the Iraq-Syria border.
The border crossing in Tanf is home to a garrison where U.S., British and Norwegian troops have been working for months with a Syrian rebel faction, named the Army of the Revolution's Commandos. It is intended to serve as a launching pad for a wide-scale attack on the eastern desert province of Dair Alzour, which is largely in Islamic State hands.
Pentagon officials have said the U.S. military would defend its forces if they were threatened in Syria by Assad's government.
"We are not increasing our role in the Syrian civil war, but we will defend our troops," Defense Secretary James N. Mattis said Thursday at 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. And that's been a going-in policy of ours for a long time."
U.S. commanders became concerned when they saw tanks, bulldozers and other heavy equipment advance without authorization into a de-confliction zone near the base where American forces were located, according to the U.S. officials, who spoke about the incident after requesting anonymity because they were not authorized to discuss the matter publicly.
The Syrian troops had also ignored a "show of force" and "warning shots" fired by coalition aircraft.
"The agreed-upon de-confliction zone agreement remains in effect," the coalition statement said.
The U.S. military does not speak with the Syrian government, so American military commanders at an air command center in Qatar called their Russian counterparts on a special hotline set up to ensure the two countries' pilots will not mistakenly run into — or fire on — one another as they conduct daily bombing runs in the skies above Syria.
After warning the Russians of the impending strikes, the U.S. planes flew near the Syrian ground forces and fired a warning shot as a show of force, the officials said. After the ground forces continued their activity, the planes carried out the airstrike, they said.
It was unclear whether the advancing forces were Syrian army troops or other pro-government allies, but they were flying Syrian flags and began constructing berms and fighting positions, officials said.
There was no confirmation of the attack by Syrian government media, but Younes Salameh, spokesman for the Army of the Lions of the East, a rebel group, said the attack had occurred about 16 miles away from Tanf on the highway linking Damascus to the Iraqi capital, Baghdad.
Mozahem Salloum, spokesman for the Army of the Revolution's Commandos, said in a Facebook post that 17 vehicles were destroyed in the attack, including four tanks. He added that support convoys were also attacked.
Other activists said eight people had been killed and two were wounded.
Pro-government activists allged the warplanes had come from Jordan, a staunch U.S. ally that has worked with the coalition to train Syrian rebel groups to fight Islamic State.
The coalition's growing presence in Tanf had not gone unnoticed by Damascus.
In recent days, pro-government troops had begun to advance toward the crossing, even while coordinating with Iraq's Popular Mobilization Units, paramilitary groups that are supported by Iran, to surround the coalition-controlled garrison from both sides.
The strike is the second time this year the U.S. military has targeted Syrian forces. It follows an April 6 attack on a Syrian government airfield with 59 Tomahawk cruise missiles in response to a chemical attack that killed dozens of civilians, including children.
The strike also comes weeks after U.S.-supported rebel factions launched a major offensive to oust the jihadis from swaths of the Syria's eastern deserts.
Special correspondent Bulos reported from Beirut and staff writer Hennigan from Washington.
3:19 p.m.: This article was updated with addition background and comments from Younes Salameh and Mozahem Salloum.
11:20 a.m.: This article was updated with staff reporting.