One fighter was killed on Monday when a roadside bomb went off near a military base in northern Syria that is used by the US-led coalition, announced the Syrian Observatory for Human Rights.
"A bomb by the side of the road exploded as a military vehicle drove by on the road from Ain Issa to the Brigade 93 base" in the northern province of Raqa, the Britain-based monitoring group said.
The blast killed at least one person and injured others of unknown nationality, it reported, in an area controlled by the coalition-backed Syrian Democratic Forces.
Spokesmen for the US-led coalition and the SDF were not immediately available for comment.
The SDF backed by the US-led coalition have expelled ISIS from much of Syria since the extremists overran large parts of the country in 2014.
They have also faced a second offensive from Russia-backed regime troops, but still hold tiny pockets in eastern Syria.
The Observatory also reported on Monday that the SDF had made wide advances against ISIS in one of the last desert areas it controls near the border with Iraq.
More than 350,000 people have been killed in Syria's war since it started in 2011 with the brutal repression of anti-regime protests.
