US-backed forces advanced on a key ISIL supply line between Syria and Iraq overnight, seizing a small airbase near the border, a monitor said on Wednesday.

US-backed forces advanced on a key ISIL supply line between Syria and Iraq overnight, seizing a small airbase near the border, a monitor said on Wednesday.

The so-called "New Syrian Army" said the operation -- launched on Tuesday -- is aimed at the severing the supply line through the Albu Kamal crossing that links areas under ISIL control in eastern Syria and Iraq's Anbar province.

The militias said the operation was coordinated with Iraqi forces who were advancing on the crossing from the other side of the border.

ISIL seized the crossing in mid-2014, when it overran swathes of territory on both sides of the border and declared a self-styled "caliphate".

The Syrian Observatory for Human Rights said the US-backed forces had advanced to within five kilometers (three miles) of Albu Kamal from both the northwest and southwest.

As the militias edged closer, ISIL beheaded five young men in the town it said were working with the US-backed forces, the Britain-based monitoring group said.

"The NSA and supporting forces captured Al-Hamdan airport, five kilometers northwest of Albu Kamal with air support from the coalition," said Observatory chief Rami Abdel Rahman.

He had no immediate word on casualties in the fighting.

The US-backed militants were trained in Jordan by US and British troops.
They already captured the Al-Tanaf border crossing between Syria and Iraq earlier this year.

ISIL is facing growing pressure from US-backed offensives in both Syria and Iraq.