Syrian regime media reported that a US-led coalition airstrike targeting positions in eastern Syria killed at least 12 pro-regime fighters, but the US military denied knowledge of it.
The Britain-based Syrian Observatory for Human Rights, which relies on a broad network of sources across Syria, said the raid hit army positions south of Albu Kamal, a town on Syria's border with Iraq.
"At least three vehicles were destroyed by the strike," said Observatory chief Rami Abdel Rahman.
He said that the 12 pro-regime fighters killed in the incident were not Syrians, but he could not provide more details on their identity.
A military source quoted by regime news agency SANA said that "some of our military positions between Albu Kamal and Hmeimeh were hit this morning in an aggression by American coalition warplanes".
SANA reported that the strikes caused only material damage and came within 24 hours of an ISIS attack on Syrian army positions in the same region.
Albu Kamal and Hmeimeh lie in Syria's eastern Deir Ezzor province, where Russian-backed Syrian troops and the US-led coalition have been waging separate offensives against the ISIS terrorist group.
A "de-confliction" line in place along the Euphrates River since last year is meant to keep the two assaults from crashing into each other.
Loyalist troops are present west of the river while the US-backed Syrian Democratic Forces (SDF) are on the east.
A military source from forces allied to Syria's regime said the strikes had targeted two regime military positions near a frontline with ISIS.
"There are no Iranian or Lebanese fighters among the dead," the source said.
ISIS lost most of its territory in Syria last year, but retained some remote desert areas and has attacked the army and allied forces in recent weeks.
On Monday, Syrian troops and allied forces ousted ISIS from the last districts it held in the capital Damascus.
After a fierce month-long battle, an evacuation deal saw the remaining jihadists bussed out of the city towards small pockets of land still held by ISIS in the Badiya, a vast desert area stretching from central Syria to its eastern border with Iraq.
The day after the transfer, ISIS fighters in the Badiya attacked a nearby regime position, leaving more than two dozen Syrian troops and allied fighters dead.
The Observatory said the ISIS fighters responsible were from the group that had just been transferred out of the Yarmuk area in southern Damascus.
