Beirut: Al-Qaeda's Syria affiliate and its Islamist allies have executed at least 56 regime troops at a military airport they recently seized in the northwest, a monitoring group said Saturday.

Al-Nusra Front and the Islamists shot dead the regime fighters, who were being held as prisoners, "execution-style" inside the Abu Duhur airport, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.

He said the killings in Idlib province had occurred earlier this week but his monitoring group -- which gathers news from sources on the ground -- confirmed them on Saturday.

Al-Nusra is a leading member of an alliance of jihadist and Islamist forces called the "Army of Conquest" that overran the Abu Duhur military airport on September 9.

When it overran the airbase, the Army of Conquest killed dozens of regime loyalists and took others captive.

The powerful coalition has seized almost all of Idlib province except for Fuaa and Kafraya, two regime-controlled villages inhabited by Shiite Muslims.

The Army of Conquest ramped up its assault on the two villages on Friday, detonating at least nine car bombs, seven of which were suicide bombers, the Observatory said.

The attack left at least 21 regime loyalists and 17 Army of Conquest militants dead.

One of the suicide bombers managed to enter Fuaa itself and killed seven civilians, including two children, Abdel Rahman said.

Fierce clashes and heavy shelling on the two villages continued on Saturday.

Fuaa, Kafraya and the rebel stronghold of Zabadani in Damascus province were at the centre of two failed attempts last month to secure broad ceasefire deals.