A Syrian rebel spokesman and activists say clashes between opposition fighters and Islamists suspected of links to the Islamic State group have raged for days in the country's south near the Israeli-held Golan Heights.

The IS group is not known to have a significant presence in southern Syria.

Isam al-Rayess, spokesman for the Free Syrian Army's southern front, said Wednesday a group known as Jihad Units ambushed some of his fighters in the Quneitra province, killing six. He said they then raised black flags similar to those carried by the IS group.

The rebels have vowed to clear the Jihad Units from the province.

The Britain-based Syrian Observatory for Human Rights says 29 people have been killed in the fighting since Monday.