"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.
Describing the truce as "shameful", the jihadist chief said that "negotiations are the ones conducted on the battlefield".
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 today just hours before the truce was due to take effect, according to the Syrian Observatory for Human Rights.
