Syrian state media, citing a military source, reported early on Monday that US-led coalition aircraft had bombed "one of our military positions" in eastern Syria, leading to deaths and injuries. The US military denied it was responsible.
The strike took place in al-Harra, southeast of Albu Kamal, it reported. There were no immediate details on casualties.
A commander in the military alliance backing Syrian President Bashar al-Assad also told Reuters that drones, "probably American," had bombed positions of Iraqi factions between Albu Kamal and Tanf and Syrian military positions.
The commander, who is not Syrian and spoke on condition of anonymity, said the strike killed and injured some Iraqi fighters, but did not give a toll.
"No member of the US-led coalition carried out strikes near Albu Kamal," Major Josh Jacques, a US Central Command spokesman, told Reuters.
The US-led coalition is supporting an alliance of Syrian Arab and Kurdish militia fighting Daesh northeast of Albu Kamal.
The Syrian army, alongside allied Iran-backed militias including Lebanon's Hezbollah and Iraqi groups, drove Daesh from Albu Kamal and its environs last year, but the jihadists have since staged attacks in the area.
US forces are also based in Tanf, southwest of Albu Kamal in the Syrian desert near the borders of Iraq and Jordan.
Last week, Assad said he regarded the United States as an occupying power in Syria and that the position of his state was to "support any act of resistance, whether against terrorists or against occupying forces, regardless of their nationality."