Syrian rebels say kill foreign ISIS fighters in undercover strike

A group of Western-backed Syrian insurgents said on Tuesday they had killed three foreign fighters - two Saudis and a Tunisian - with ISIS in an undercover raid into an area under the control of the ultra-radical militants.

An alliance of rebel groups known as Jabhat al-Shamiya (Levant Front) operating in the northern Aleppo countryside near the border with Turkey said the operation was conducted by their clandestine security unit and was the first of its kind.

They pledged further commando strikes in the future.

"We promise Baghdadi (ISIS leader) more of these security operations in the heart of areas under the control of his fighters," a statement by the Levant Front said.

The region has witnessed intensified fighting in Syria's civil war between Western-backed rebels and Islamist insurgents who have gained further ground, threatening the fall of most of the northern countryside of Aleppo into their hands.

ISIS has now encircled the rebel-held town of Marea, 20 km (12 miles) from the Turkish border, an area that also falls within a "safe zone" Turkey said last month it would set up in northern Syria to help keep ISIS at bay.

The militants have since May made advances that threaten a major supply route used by rival rebels into the city of Aleppo.

On Tuesday, Western-backed rebels accused Islamic State of firing at least 15 mustard gas mortars during heavy shelling of Marea. A loss of Marea would make it harder for Turkey and the United States to open a new front in the region against Islamic State in coordination with rival rebels on the ground.

Western-backed rebels have sent in reinforcements from other parts of Aleppo province to try to beat back the jihadists, according to a Levant Front fighter.

ISIS holds large swathes of territory across Syria and Iraq, and has advanced in other areas of Syria in recent months, capturing the central city of Palmyra.

The militant group is fighting rival insurgents, the Syrian military and Kurdish regional forces alike in Syria's four-year-old civil war.