They said eight Islamic State fighters and four of their own men died.
An attack at midnight on a heavily defended base near the al Tanf border crossing involved at least one explosive-laden vehicle that rammed an entrance to the base. At least two U.S.-backed rebels were killed and scores wounded, a rebel source said.
The militants also staged a suicide attack on a convoy of rebel fighters from the Western-backed Osoud al Sharqiya rebel group, who had sent reinforcements from their outpost near the Rukban refugee camp further south west. Two of the fighters in the convoy were killed in the ambush.