US-backed forces said Friday they had captured a leader of ISIS in eastern Syria where the Kurdish-led fighters have been battling the militants.
A statement by the Syrian Democratic Forces (SDF) identified the suspect as Osama Oweid Saleh and described him as "one of the most dangerous terrorists" of ISIS.
But Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights war monitor, disputed the claim.
Mr Abdel Rahman said Saleh was merely "a former local security official" in the eastern Deir Ezzor province.
In its statement the SDF said that Saleh "was a security official for the terrorists [ISIS] in Deir Ezzor and took an active part in planning and implementing more than 40 terrorist operations" for the extremist group.
It also said that he was "a security official" in other parts of Syria for ISIS, including in the former militant bastion of Raqqa.
Saleh, it said, was ambushed by SDF fighters and captured on November 22 in the Deir Ezzor countryside.
Mr Abdel Rahman said that Saleh "could be a member of an ISIS sleeper cell".
The SDF, an alliance of Kurdish and Arab fighters, is seeking to expel ISIS from a pocket of land in the Deir Ezzor province near the Iraq border.
The Kurdish-led forces have spearheaded the US-backed fight against ISIS in Syria.
On Monday the Observatory reported that the SDF suffered record fatalities in an assault by ISIS as holdout militants kept up a fierce defence of their last Syrian redoubt.
It said a total of more than 200 people have been killed since around 500 ISIS fighters burst out of the fog shrouding the area in eastern Syria near the border with Iraq to launch their deadly assault last Friday.
Ninety-two of the dead were SDF fighters while at least 61 extremists and 51 civilians, mostly their relatives, also died in the violence, it said.