The head of Syria's powerful Al-Qaeda branch on Friday urged opponents of President Bashar al-Assad to reject a ceasefire due to begin at midnight and instead intensify attacks on the regime.

"Beware of this trick from the West and America because everyone is pushing you to go back under the thumb of the oppressive regime," Mohammad al-Jolani, the head of Al-Nusra Front, said in an audio message.

"Fighters in Syria, willingly arm yourselves, intensify your attacks and have no fear of their troops and their aircraft," Jolani added.

The ceasefire agreement brokered by Russia and the United States marks the biggest diplomatic push yet to help end Syria's violence, but has been plagued by doubts after the failure of previous peace efforts. 

Parties to the deal -- which allows fighting to continue against jihadists -- will have to deal with the complexity of Syria's battlefields where moderate and Islamist rebel forces often fight alongside extremists groups such as Al-Nusra.

Russia carried out intense raids on rebel bastions across Syria Friday just hours before the truce was due to take effect, according to the Syrian Observatory for Human Rights.

Syria's army said this week it would exclude Daraya, an important rebel town near Damascus from the cessation of hostilities because forces there including Al-Nusra fighters.