More than 100 pro-regime fighters were killed, the official added.
The SDF and the coalition targeted the attacking forces with air strikes and shelling after "20 to 30 artillery and tank rounds landed within 500 metres (yards) of the SDF headquarters location", a U.S. official said.
The clash came against a backdrop of escalating tensions between Washington and Damascus over an uptick in the suspected use of chemical weapons by the regime and allied militia.Meanwhile, regime warplanes rained bombs for a fourth consecutive day killed 38 on the rebel-held enclave of Eastern Ghouta, where the death toll soared to more than 200 since Monday.
Syrian state news outlet SANA said those killed were primarily battling the Islamic State, although it admitted an attack against the SDF took place. Additionally, Turkey has threatened to launch a military operation against Manbij, Syria, a place where USA troops do work with local Kurdish and Arab fighters.
Recently, a funeral was held at the town of Al-Suqaylabiyah for ten of the ISIS Hunters soldiers killed in the United States airstrikes - many of them were Christians.
A large-scale attack by over 500 Syrian troops aligned with Bashar al-Assad on a base used by USA troops and local United States allies underscored the challenges faced by the Trump administration as it seeks to implement its long-term strategy for Syria, with both the Russian-backed regime and North Atlantic Treaty Organisation ally Turkey recently conducting or threating attacks against America's primary ally in Syria.
However, America's North Atlantic Treaty Organisation ally Turkey says those same Kurdish groups are linked to Kurdish separatists who have fought a decade-long insurgency against the Turkish government.Shortly after Tillerson's speech and an announcement by the U.S. military that it meant to work with the Syrian Democratic Forces to establish a border security force in order to secure areas liberated from ISIS, Turkey launched a major military operation targeting Kurdish groups in Afrin, Syria.While the United States does not support groups in Afrin, they are linked to American allies elsewhere in Syria. It also lowers (in only a very small way) the likelihood of a confrontation with the US if these forces come into contact.More news: Apple's pricey HomePod sounds great but exacts some trade-offs
CNN also reported that HIMARS and MQ-9 drones were used in the attack.
According to Russia's Ministry of Defense, the advancing pro-regime forces had not coordinated with Russian officials in the area. "The incident was caused by uncoordinated reconnaissance and search operations plans of the Syrian militias with the Russian command task force in the locality of Salhiyah". America also has 2,000 soldiers on the ground in Syria with the SDF.
Washington is determined to deny the Syrian government control over these resources and to that end has sought to carve out a U.S. zone of control covering roughly 30 percent of the country, while cutting off its borders with Turkey and Iraq. Other officials said the USA also used Air Force AC-130 gunships and Army AH-64 Apache attack helicopters to strike the pro-government fighters.
"First of all, how could a 500-strong unit attack a headquarters with tank and artillery support and, as a result, inflict an injury on one counter-attacker?" asked Russian Foreign Ministry spokesperson Maria Zakharova.
"How, within such a short period of time, could a decision have been made to open massive fire for effect on Syrian armed forces?" she continued.
A USA military airstrike has killed over 100 Pro-regime soldiers fighting for the Assad government, against anti-Assad rebels. "ISIL is still there, and our mission is still to defeat ISIL", Pentagon spokeswoman Dana White said Thursday. ".we told them, they knew what was happening".
The river acts as a de-confliction line between the coalition and Russian and Syrian forces.