Syrian rebels prepare to launch an anti-tank missile (file)
Syrian rebels prepare to launch an anti-tank missile (file)Reuters

At least 42 fighters were killed in 24 hours of fierce fighting between Islamist rebels and the Islamic State group in Syria's Damascus province, a monitoring group said Wednesday.

"At least 30 Islamist rebels and 12 fighters from ISIS were killed in fighting since Tuesday" in the hilly region of Qalamun, the Syrian Observatory for Human Rights said, according to AFP.

Qalamun is divided into a western portion, which borders Lebanon and is mostly controlled by the regime and its ally, the Lebanese Shiite terror movement Hezbollah.

The eastern sector has seen intense clashes between rebels and ISIS, and is strategic because it borders the "badiya," the Syrian steppe.

These plains are used by rebels to transport weapons from the Turkish border to the north and the Jordanian frontier in the south.

According to the Britain-based Observatory, ISIS has already cut off one of these routes and aims to take more to "suffocate" the rebels.

Meanwhile, at least 11 people were killed in regime bombardment of two towns in Damascus province.

"It is likely that there were even more killed in the attack, and the number will increase due to people in a critical condition," said Observatory head Rami Abdel Rahman, according to AFP.

The fighting among rebel groups is nothing new, as different rebel groups have been fighting one another constantly in addition to fighting the Bashar Al-Assad regime.

Al-Nusra Front, a rebel group which has been an official branch of Al-Qaeda since April 2013 and dominates a swathe of northwest Syria, has clashed with Western-backed rebels and its jihadist rival the Islamic State (ISIS) group.

Last November, Al-Nusra and ISIS temporarily agreed to work together in order to combat the international airstrikes against ISIS in Iraq and Syria.