A commander in a pro-government alliance meanwhile told Reuters news agency that several Iraqi militiamen had been killed in strikes by drones, "probably American", on positions between Albu Kamal and al-Tanf - an area 230km (145 miles) to the south-west where some usa military personnel are based.
"At 22:00 last night a USA plane hit a fixed headquarters of the Popular Mobilisation Forces' 45th and 46th brigades defending the border strip with Syria using two guided missiles which led to the martyrdom of 22 fighters", it said in a statement.
It said the raid took place "700 meters (700 yards) inside Syria".
A Middle East correspondent said: "Syrian forces killed in a global coalition raid on a military site south of Albukamal".
Syrian state media said on Monday that US -led coalition aircraft had bombed "one of our military positions" in eastern Syria, leading to deaths and injuries, but the USA military denied carrying out strikes in the area.
A US-backed alliance of Kurdish and Arab fighters and Russia-supported regime forces are carrying out separate operations against those IS-held pockets.
Washington said the strikes were in retaliation for an attack on its own personnel and SDF forces.More news: Brexit: May Un-Surrenders to Tory Remainers, Faces Parliamentary Rebellion
The United States acknowledges that it has about 5,200 troops in Iraq that are part of a coalition fighting Islamic State militants.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said it had documented the deaths of 38 non-Syrian militia fighters, but that it did not know whether they were Iraqi or Lebanese nationals. Reuters could not independently verify the Observatory's report of casualties.
"We will act against these efforts anywhere in Syria", Israeli Prime Minister Benjamin Netanyahu said Sunday at a cabinet meeting.
Throughout Syria's seven-year war, Israel has carried out scores of strikes within the neighboring country against what it describes as Hezbollah or Iranian targets.
ISIS fighters are close to losing their small pockets in east Syria after being besieged in the two provinces of Hasaka and Deir Ezzor, particularly in areas located east of the Euphrates River.
Netanyahu spoke with US Secretary of State Mike Pompeo and Russian President Vladimir Putin over the weekend, two of the key players in influencing Israel's policy and actions over Syria.
Damascus says it considers the United States an occupying force in Syria and supports any resistance against USA forces.