The coalition said "apparent" Russian attempts to stop pro-Assad forces from moving toward Tanf, as well as warning shots and a show of force, had failed. American officials and Syrian activists said the strike hit in the desert near the border with Jordan, though it was unclear if it struck the Syrian army or just militias allied with the government.
The region around Tanf, where the borders of Jordan, Syria and Iraq meet, has been considered a de-conflicted zone, under an agreement between the U.S. and Russia. Speaking to reporters, Defense Secretary Jim Mattis said the U.S. will defend its troops in case of "aggressive" steps against them. He was asked if the airstrike increases the U.S. role in the Syrian war.
"We are not increasing our role in the Syrian civil war, but we will defend our troops," Mattis said. "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 "defensive" strike was also an apparent signal to Assad to keep his forces out of a zone where U.S.-backed rebels are fighting the Islamic State group. "This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south ... were unsuccessful, a coalition aircraft show of force, and the firing of warning shots," the U.S.-led coalitions aid. It said coalition forces have been operating in the area "for many months training and advising vetted partner forces" in the battle against IS.
The U.S. strike marks a new approach in what has become an intensely crowded and complicated war zone. Thursday's strike was the coalition's first on pro-Assad forces in the battlefield. The coalition had so far kept its military operations focused on Islamic State militants and al-Qaida-linked groups.
Last month, the U.S. fired 59 missiles at a government air base in central Syria as punishment for a chemical attack blamed on Assad's forces that killed nearly 90 people. An increasingly visible U.S. role in Syria has also raised the possibilities of friction with the various forces on the ground. The U.S. is backing Syrian Kurdish forces who are also fighting IS to the country's east. US troops have sent patrols in the area to act as a buffer between Turkish troops and the Kurdish fighters. Turkey views the U.S-backed Kurdish fighters as an extension of its own insurgent group.
In recent days, near the border with Jordan, another set of U.S-backed rebel fighters have been on a collision course with government troops in the area of Tanf. The government launched a new offensive in recent days in the area, and activists say pro-government militiamen, mainly from Iran and the Lebanese Shiite militant Hezbollah group, have deployed there aiming to secure the main highway that runs from Damascus to Baghdad and beyond, to Tehran.
Tensions have been building as part of a race for control of territory stretching from the provincial capital of Deir el-Zour in northeastern Syria to the Iraq border. The area gained attention as the battle for the Iraqi city of Mosul escalated in recent weeks. An estimated 10,000 IS fighters uprooted from Mosul are believed to be massing in the border area.
The U.S. officials said the American airstrike hit the pro-Syrian government forces as they were setting up fighting positions in a protected area near Tanf. They said a tank and a bulldozer were also hit.
One official said the pro-regime forces had entered a so-called "de-confliction" zone without authorization and were perceived as a threat to U.S.-allied troops there. The officials weren't authorized to speak publicly on the matter and demanded anonymity.
A Syrian opposition media group, the Palmyra News Network, said the attack occurred at the Zarka juncture, about 27 kilometers (17 miles) from the border, destroyed a number of vehicles and caused casualties. The Syrian Observatory for Human Rights also said the strike destroyed vehicles and killed eight militiamen. There was no immediate comment from the pro-government side.
In September 2016, the coalition erroneously struck at Syrian government troops in Deir el-Zour, killing over 90 soldiers. The U.S. at the time said it was a mistake, as it was targeting IS positions.
Further to the north, IS militants on Thursday attacked several government-held villages in central Syria, capturing at least one and killing 52 people. The dead included more than two dozen women and children, some of whom were beheaded, as well as Syrian troops, according to state media, medical officials and an opposition monitoring group.
The U.S.-backed Syrian Kurdish-rebels are closing in on Raqqa, the de facto capital of IS in Syria. That battle has already caused clashes between government forces and Syrian rebels and raised concern of pro-government militias making a bid for controlling the border with Iraq.
The IS attack in the central Hama province, meanwhile, targeted villages where most residents belong to the Ismaili branch of Shiite Islam, raising fears of massacres such as those the Islamic State group carried out in other minority communities in Syria and Iraq.
The villages are located near the town of Salamiyeh and the highway that links the capital, Damascus, to the northern city of Aleppo, but state media said traffic was not affected. Media reports and doctors in the area said some of the killed, which included women and children, were beheaded and others dismembered. IS extremists are notorious for mutilating bodies of their adversaries, particularly members of other sects than Sunni Islam.
The militants stormed homes in the southern part of Aqareb al-Safi village before government forces pushed them back into the desert, the state news agency SANA reported. The head of the National Hospital in Salamiyeh, Dr. Noufal Safar, said it received 52 bodies, including 11 women and 17 children.
Some of the bodies were badly mutilated, beheaded or had their limbs severed but "most appear to have died as a result of gunfire," Safar told The Associated Press by telephone. Rami Razzouk, a coroner at the hospital who inspected the bodies, said those of children were brought in mostly dismembered while the men had died from shelling or heavy machine-gun fire. He said at least nine children were beaten on the head with heavy objects such as bricks or stones.
The Observatory also said that 52 people were killed in the fighting, with the dead including 15 civilians, 27 Syrian soldiers and 10 unidentified people. Razzouk said 120 people were wounded; SANA said 40 were wounded.
The IS-linked Aamaq news agency said the militants captured villages of Aqareb al-Safi and Mabouja. It identified residents as members of Assad's Alawite sect, an off-shoot of Shiite Islam. The Sunni extremists view Shiites as apostates deserving of death.
IS has massacred thousands of Shiites and other opponents in Syria and Iraq, often boasting about the killings and circulating photos and videos of them online. Aamaq claimed that 100 Syrian troops and pro-government gunmen were killed in the fighting.
"Dozens of people are missing but it is not clear if they were kidnapped" by IS, the Observatory's chief Rami Abdurrahman said.
Baldor reported from Washington. Associated Press Writers Robert Burns in Washington and Bassem Mroue and Hashem Osseiran in Beirut contributed to this report.