Al-Qaeda-linked Syrian jihadist group Al-Nusra Front on Sunday vowed revenge strikes against villages from President Bashar al-Assad's community over claims his forces used chemical weapons.

AFP reported that Syria's main opposition body, the National Coalition, has accused Assad's forces of "massacring" more than 1,300 people in chemical weapons attacks on the outskirts of Damascus on Wednesday, which his regime has denied.

"The Alawite villages will pay the price for each chemical rocket that struck our people in Damascus," Al-Nusra front chief Abu Mohammed al-Jawlani said in an audio message posted on the Internet and on his Twitter account, adding that "one thousand rockets will be used for this purpose".

"It is a debt towards... our relatives in the eastern Ghouta," he said.

"This regime attacked the eastern region (of Damascus province) with dozens of chemical rockets that killed hundreds of children, women and men," Jawlani said, calling on opposition fighters across the country to take revenge.

Jawlani also suggested that the revenge attacks could take place as soon as Sunday, calling on "the soldiers of Al-Nusra" to "spread their fire... before the end of the day and the setting of the sun".

Assad hails from the Alawite community, an offshoot of Shiite Islam unlike most Sunni rebel groups fighting to topple him, including Al-Nusra, whose chief in March pledged allegiance to Al-Qaeda.

Meanwhile, French President Francois Hollande said Sunday that evidence indicated Syrian President Bashar al-Assad's regime had carried out chemical weapons attacks.

Hollande said there was "a body of evidence indicating that the August 21 attack was chemical in nature, and that everything led to the belief that the Syrian regime was responsible for this unspeakable act".

In Israel, president Shimon Peres called Sunday for an international effort to "take out" chemical weapons in Syria after claims President Bashar al-Assad's regime used them in a deadly attack.

"The time has come to make a joint effort to take out all the chemical weapons from Syria," Peres said, without elaborating if he envisioned this being achieved through military strikes or otherwise.

Doctors Without Borders said that 355 people of the thousands treated at three hospitals died with "neurotoxic" symptoms stemming from attacks on Wednesday near Damascus.