Coalition troops known as VSO, lead by the US, said the group had launched an attack on Saturday on the Tanf garrison on the Syrian-Jordanian border using a vehicle-borne IED, and 20-30 fighters followed with a ground assault and suicide vests.
It is believed that at least one explosive-laden vehicle tried to ram an entrance to the base.
The attack came days after Donald Trump launched airstrikes on Syria late on Thursday – in response to a chemical weapons attack on civilians allegedly carried out by Bashar al-Assad's government.
The evacuation of Aleppo (archive image)
A press release issued by the coalition read: "Coalition and partnered forces engaged and defended the ISIS attack with direct fire before destroying enemy assault vehicles and the remaining fighters with multiple coalition airstrikes.”
Initial reports indicate the assault did not result in any coalition casualties, a coalition official said, who added that some US forces were at the base at the time.
The coalition said: ”In Southern Syria, vetted Syrian opposition forces focus on conducting operations to clear ISIS from the Hamad Desert and have been instrumental in countering the ISIS threat in southern Syria and maintaining security along the Syria-Jordan border.”
ISIS-affiliated Amaq news agency said two suicide attacks were conducted by its fighters near Tanf on “positions of Syrian groups supported by US.” It gave no details.
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In a separate incident, 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.
A senior rebel source from Osoud al Sharqiya who requested anonymity, said ISIS “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.”
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.
An al-Tanf immigration centre
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 southeast of Damascus, all the way to the borders with Iraq and Jordan, a desert area known as the Badia.
A rebel official, Said Seif, with the western-backed Shahid Ahmad Abdo group that operates in the area, told Reuters: “Their message is we are still present in the area and have not withdrawn and we still target the FSA [Free Syrian Army].”
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.
An immigration centre at al-Tanf
A western intelligence source told Reuters that US 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 ISIS 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.
ISIS had seized large swaths of land in its quest to create a caliphate that stretched from western Syria to eastern Iraq but is now being steadily pushed back.
Russia bombed the Tanf garrison in June 2016, but there were no US forces there at the time. During that incident the coalition sent aircraft in an attempt to prevent airstrikes.