Al Qaeda affiliated Nusra Front called for escalation of fighting against the Syrian regime and its allies, adding to the dangers facing an agreement to implement cessation of hostilities set to start on Saturday.
The Syrian regime and opposition groups have agreed to take part in a US-Russian "cessation of hostilities" deal that is due to start at midnight (2200 GMT) on Friday.
The cessation deal does not include DAESH and the Al Qaeda' Syrian affiliate Nusra Front, as the Syrian regime and its Russian allies say they will not stop combat against those groups. Other moderate rebels say they fear this will be used to justify attacks on them.
Nusra's leader, Abu Mohamad al Golani, said in an audio message on Orient News TV that militants should "strengthen your resolve and intensify your strikes, and do not let their planes and great numbers (of troops) scare you."
Unlike DAESH, which seized territory in central and eastern Syria, the Nusra front is widely dispersed in opposition-held areas in the west, and any escalation would spark the risks of the deal collapsing.
Heavy air bombardment continues as the deadline for the the cessation of hostilities approached, massive air strikes were reported to have hit opposition-held areas near Damascus while fighting intensified across much of western Syria.
President Vladimir Putin said Russia had received information that all parties were ready to abide by the cessation of hostilities, Russian news agencies reported.
However he stressed that, combat actions against DAESH and the Nusra Front would continue.
"I would like to express the hope that our American partners will also bear this in mind ... and that nobody will forget that there are other terrorist organisations apart from Islamic State," he said in Moscow.
The United Nations hopes the cessation of hostilities will provide a chance to resume peace talks in Geneva, which would collapsed this month before they start.
Peace talks could resume on March 7. In New York, a Russian Foreign Ministry official said. UN Security Council would vote later on Friday on a resolution to endorse the planned pause in combat.