Free Syrian Army fighters clean their weapons and check ammunition at their base on the outskirts of Aleppo, Syria.

Syrian rebel fighters began an operation on Tuesday to capture a town held by ISIS near the Iraqi border, a senior rebel commander said, opening a new front against the militants who are also being attacked by US-backed forces in the north.

Capturing the town of Al-Bukamal from ISIS would hamper its ability to move between the Iraqi and Syrian territories of its self-declared caliphate.

The commander said the rebels, including fighters of the New Syria Army, had secured the desert approaches to the town. Another rebel source said they were now inside its administrative boundaries.

The New Syria Army was formed some 18 months ago from rebel fighters driven out of eastern Syria by ISIS as it expanded its territories in mid-2014 following the capture of the Iraqi city of Mosul.

In a video posted by the group, a commander was filmed being driven in a military vehicle in an undisclosed desert location and saying his troops were heading to “liberate the town.”

The New Syria Army has received support from the US-backed alliance that is battling ISIS in Syria and Iraq.

Its main base is in al Tanf in southern Syria which was hit twice earlier this month by Russian air strikes, even after the US military used emergency channels to ask Moscow to stop after the first strike, according to U.S. officials.

The US-led campaign against ISIS in Syria has gone up a gear this month, with an alliance of militias including the Kurdish YPG launching a major offensive against the group in the city of Manbij in northern Syria.