IS group militants launched two suicide attacks on U.S.-backed Syrian rebels near the border with Iraq, leaving at least 12 dead in the fighting and many wounded, rebel sources said on Sunday.
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.
Islamic State "staged a suicide attack and there were clashes inside Tanf. Two were killed and several injured. They also attacked our convoy but it's over and matters are under control," said a senior rebel source from Osoud al Sharqiya who requested anonymity.
Islamic State-affiliated Amaq news agency said two suicide attacks were conducted by its fighters near Tanf on "positions of Syrian groups supported by America." It gave no details.
U.S.-led coalition planes were involved in the operation to track down the militants who staged the hit-and-run attack and apparently fled, a rebel commander involved in the operation said.
The Pentagon estimated between 20 and 30 Islamic State fighters, some wearing suicide vests, were involved in the ground assault.
"Coalition and partnered forces defended against the ISIS attack with direct fire before destroying enemy assault vehicles and the remaining fighters with multiple coalition air strikes," the Pentagon said in a statement.
Both Tanf and Rukban are near the joint Syria-Iraq-Jordan border. Osoud al Sharqiya, one of the main groups in that area fighting Islamic State, is part of the Free Syrian Army of rebels financed and equipped by a Western coalition.
Jordan, a U.S. ally, backs the moderate rebel groups aligned with the so-called Southern Front supported by an Arab-Western coalition, who are trying to prevent opposition-held southern Syria from falling to Islamic State.
The rebels took the border crossing of Tanf last year from the militants and tried unsuccessfully to drive them out of the Syrian border town of Bukamal on the Euphrates, further northeast, a major supply conduit for Islamic State between its strongholds in Iraq and Syria.
In recent weeks, the militants in the Syrian desert near the Jordanian border have regrouped further north to reinforce their Raqqa stronghold, after major defeats in Syria and Iraq.
The attacks were meant to show the militants were still capable of waging hit-and-run operations against the Western- backed rebels who have recently seized a swathe of territory stretching from the town of Bir Qasab, some 50 km (30 miles) southeast of Damascus, all the way to the borders with Iraq and Jordan, a desert area known as the Badia.
"Their message is we are still present in the area and have not withdrawn and we still target the FSA," Said Seif, a rebel official with the Western-backed Shahid Ahmad Abdo group that operates in the area, told Reuters.
Western intelligence sources have worried for months that militants fleeing from their main urban strongholds of Raqqa in Syria and Mosul in Iraq could find a safe haven in the region.
A Western intelligence source told Reuters that U.S. and British special forces are expanding the Tanf base to use it as a major launching pad for operations in coming months to oust militants from Bukamal, a major militant stronghold.
Diplomats say plans are under way to stage new coalition strikes on Islamic State fighters in the south, including an area to the west of the southern city of Deraa. Militants entrenched in the Yamouk River Valley near the Israeli border have recently made gains.