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 United States military personnel are based.
"US planes fired two guided missiles at a fixed position of Hashed al-Shaabi units on the border with Syria, killing 22 fighters and wounding 12", the Iran-backed Hashed said.
US forces are also based in Tanf, southwest of Albu Kamal in the Syrian desert near the borders of Iraq and Jordan.
It said the raid took place "700 meters (700 yards) inside Syria".
"Thirty-eight non-Syrian fighters from regime loyalist militias were killed in the night-time raid on Al-Hari, on the Syrian-Iraqi border", said Observatory head Rami Abdel Rahman. The Iraqi officials spoke on condition of anonymity because they were not authorized to speak to the media.
The Iraqi PMF have demanded an explanation from the United States over the airstrikes, but USA defense officials have repeatedly stressed that neither US nor allied aircraft attacked PMF or Syrian regime forces. "We are concerned about the perception that we conducted this strike, particularly because we want to avoid any possible retaliation against our forces in Iraq or Syria".
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Syrian President Bashar Assad has blasted the US presence in Syria, referring to it as an occupation.
The Syrian state TV report said the airstrike occurred around midnight in the village of al-Hari, to the southeast of the border town of Boukamal, and was carried out by the US-led coalition battling the Islamic State group. Reuters could not independently verify the Observatory's report of casualties.
Asked about the reported air strikes, an Israeli military spokeswoman said: "We do not comment on foreign reports".
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.
Israel, alarmed about the clout of arch enemies Iran and Hezbollah, has been pressing Russia, Assad's other key ally, to make sure they do not entrench their military sway in Syria.
Assad said his government would "support any act of resistance, whether against terrorists or against occupying forces, regardless of their nationality", according to Reuters.
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