American warplanes have bombed pro-government forces in eastern Syria, killing scores of fighters after what U.S. officials described as an unprovoked assault on allied Syrian militiamen and their U.S. advisors.
The rare U.S. strike against forces loyal to Syrian President Bashar Assad came overnight Wednesday amid a surge of violence in the country. It underscored the difficulty of maintaining a U.S. military presence there to fight Islamic State remnants without getting mixed up in the broader conflict.
Regional experts had warned that the collapse of Islamic State's self-declared caliphate last year would bring to a head the conflicting interests of various local groups and their international backers who once had a common enemy in the militants. At the same time, it has freed up pro-Assad forces to go after the remaining pockets of territory that have fallen from government control since the start of an uprising in 2011.
This week alone, heavy airstrikes are reported to have killed scores of people in the eastern Ghouta region, the last major rebel stronghold near the capital, Damascus. An escalation of fighting in the northwestern province of Idlib has also displaced about 300,000 people since December.
United Nations officials on Tuesday sounded the alarm about a dramatic deterioration in humanitarian conditions and called for an immediate cessation of hostilities for at least a month to allow aid workers to reach besieged population centers.
Late Wednesday, a battalion-sized element of pro-government fighters, backed by tanks, artillery, multiple-launch rocket systems and mortars, launched what appeared to be a coordinated attack on a base belonging to the Syrian Democratic Forces in Dair Alzour province, said Col. Ryan Dillon, a spokesman for the U.S.–led coalition battling Islamic State in Iraq and Syria.
Coalition advisors were stationed with the Syrian Democratic Forces, or SDF, an alliance of Kurdish and Arab militias that has proven one of the most effective forces against the militants. After about 20 to 30 artillery and tank rounds landed within 500 yards of the base, the SDF and coalition responded with air and artillery strikes, Dillon said by phone from Baghdad.
As many as 100 of the attackers were killed, according to one U.S. estimate. But Dillon said coalition forces were still assessing the extent of casualties on the government side. No U.S. or coalition forces were injured or killed in the incident, he said; the SDF said two of its fighters suffered minor injuries.
Pentagon spokeswoman Dana W. White said the United States was not looking for a conflict with the Syrian government. "Our forces have the inherent right of self-defense," she said.
The Syrian government labeled the incident a "massacre" and accused the U.S. of supporting terrorism.
Tension has increased sharply since the Trump administration announced that it intends to maintain a presence in Syria to ensure Islamic State does not make a comeback in areas now under the control of the SDF and counter Iranian influence.
The Syrian government and its allies, Russia and Iran, have declared the U.S. presence illegal. Turkey, meanwhile, views the U.S.-backed SDF as a front for Kurdish militants who have waged a decades-long insurgency on its side of the border.
Coalition and SDF forces had over the past week observed a buildup of pro-government fighters near Khusham, a town about five miles east of a so-called de-confliction line that was established by the U.S. and Russian militaries, which support opposing sides in the war.
Under the arrangement, the Syrian government forces and their allies are supposed to stay on the west side of the Euphrates River while U.S.-backed SDF fighters continue to go after Islamic State fighters to the east.
But on Wednesday, the forces began to advance on Khusham and the adjacent area of Tabiyah, where some of Syria's largest oil and gas fields are located, setting off an intense firefight that lasted until the earning morning, said Lulwa Abdullah, an SDF spokeswoman.
"Our forces broke their attack, and the international coalition attacked some of their headquarters and heavy armor," she said. "Till now, we don't know the reason why they attacked, but we won't give up a single inch of Syrian lands that had been liberated by the pure blood of our martyrs."
U.S. officials suspect the pro-government forces were attempting to seize terrain that the SDF recaptured from Islamic State in September, including the Khusham oil fields.
U.S. commanders contacted their Russian counterparts about the buildup and stayed in touch with them during and after the clash, Dillon said. "Pro-regime vehicles and personnel who turned around and headed back west were not targeted," he said.
The Russian Defense Ministry said a pro-government militia was carrying out a reconnaissance operation against militants who had been shelling the government's positions but neglected to inform the Russian military about its plans.
"The recent incident once again shows that the United States' illegal military presence in Syria is actually aimed at taking control of the country's economic assets and not at fighting against the ISIL international terror group," the ministry said in a statement, using an acronym for Islamic State.
Neither side identified the pro-government forces. But Rami Abdul Rahman, who heads the pro-opposition Syrian Observatory for Human Rights, said they were members of the Fatemiyoun Brigade, a group composed of Shiite Muslim Afghans recruited by Iran to fight in Syria.
The latest clashes add yet another layer of complication to a deadly conflict that is about to enter its seventh year.
In Ghouta, once Syria's breadbasket and one of several Russian-brokered "de-escalation zones," a government siege has intensified with airstrikes that have killed at least 460 people since Dec. 29, including 54 on Thursday, the observatory said.
Activists published image after image showing the strikes' aftermath, with bloodied residents walking away from rubble-filled areas in a daze. Workers with the Syrian Civil Defense, a pro-opposition rescue team known as the White Helmets, flooded social media with horrific videos depicting their frantic efforts to reach casualties under ravaged structures.
At least four other people were killed in airstrikes Thursday in Idlib, another supposed de-escalation zone that has witnessed a jump in violence in recent weeks.
Turkey, meanwhile, has opened a new offensive to drive Kurdish forces from areas along its border with Syria. The operation has been confined to the western area of Afrin, but Turkish President Recep Tayyip Erdogan has repeatedly vowed to press east to Manbij, where U.S. troops are stationed.
Though it is unclear whether U.S. forces would be willing to use the level of force deployed on Thursday against troops from Turkey, a NATO ally, SDF officials are confident that the coalition would come to their defense.
U.S. officials have described the Turkish offensive as an unwelcome distraction from the fight against Islamic State, which still has an estimated 1,000 active fighters in the Euphrates River valley to the east of Syria.
Noah Bonsey, a senior analyst for the International Crisis Group on Syria, said it was not surprising if pro-government forces had seized the opportunity to make a push for oil-rich areas in the east of the country as SDF fighters rushed to the aid of the Kurds in the west.
"It's premature to say that this conflict is winding down," he said. "Elements of the conflict are winding down, and other elements are escalating."
Bulos is a special correspondent.
4:12 p.m.: This article has been updated to add reaction from Russia and details about the fighting elswhere in Syria.
11:30 a.m.: This article was updated throughout with staff reporting.