US-backed forces say IS security leader seized in Syria
The suspect was identified as Osama Oweid Saleh and described as "one of the most dangerous terrorists of the Daesh (IS) group" the SDF statement said.
But Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights war monitor, disputed the claim.
Abdel Rahman said Saleh was merely "a former local security official" in the eastern Deir az-Zour province.
In its statement the SDF said that Saleh "was a security official for the terrorists (IS) in Deir az-Zour and took an active part in planning and implementing more than 40 terrorist operations" for the jihadist group.
It also said that he was "a security official" in other parts of Syria for IS, including in the former jihadist bastion of Raqqa.
Saleh, it said, was ambushed by SDF fighters and captured on November 22 in the Deir az-Zour countryside.
Abdel Rahman told AFP that Saleh "could be a member of an IS sleeper cell".
The SDF, an alliance of Kurdish and Arab fighters, is seeking to expel IS from a pocket of land in the Deir az-Zour province near the Iraq border.
The Kurdish-led forces have spearheaded the US-backed fight against IS in Syria.
On Monday the Observatory reported that the SDF suffered record fatalities in an assault by IS as holdout jihadists 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 IS 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 jihadists and 51 civilians, mostly their relatives, also died in the violence, it said.