Syria's branch of Al-Qaida, one of its most powerful Islamist rebel groups, called for an escalation in fighting against the government and its allies, adding to the dangers facing an agreement to halt fighting set to start on Saturday.
The government and rebel groups have agreed to take part in a U.S.-Russian "cessation of hostilities" accord that is due to begin at midnight. Warring parties had been required to accept by noon.
Under the measure, which has not been signed by the Syrian warring parties themselves and is less binding than a formal ceasefire, the government and its enemies are expected to stop shooting so aid can reach civilians and peace talks begin.
The truce does not apply to jihadist groups such as Islamic State and the Al-Qaida-linked Nusra Front, and the Damascus government and its Russian allies say they will not halt combat against those militants. Other rebels seen as moderates by the West say they fear this will be used to justify attacks on them.
The Nusra Front on Friday urged insurgent groups to intensify their attacks against President Bashar al-Assad and his allies.
Nusra's leader, Abu Mohamad al-Golani, said in an audio message on Orient News TV that insurgents should "strengthen your resolve and intensify your strikes, and do not let their planes and great numbers (of troops) scare you."
Unlike Islamic State, which controls defined areas of territory in central and eastern Syria, the Nusra Front is widely dispersed in opposition-held areas in the west, and any escalation would add to the risks of the truce collapsing.
Nusra is bigger than nearly all the factions taking part in the cessation, with fighters across western Syria.
As the deadline for the cessation of hostilities approached, heavy air strikes were reported to have hit rebel-held areas near Damascus while fighting raged across much of western Syria.
The Syrian government has agreed to the cessation plan. The main opposition alliance, which has deep reservations, said it would accept it for two weeks but feared the government and its allies would use it to attack opposition factions under the pretext that they were terrorists.
President Vladimir Putin said Russia had received information that all parties expected to take part in the cessation of hostilities had said they were ready to do so, Russian news agencies reported.
Putin stressed that combat actions against Islamic State, the Nusra Front and other groups which the Syrian government regards as terrorists 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 pause in fighting will provide a breathing space to resume peace talks in Geneva, which collapsed this month before they began.
A Russian Foreign Ministry official said the Geneva talks could resume on March 7. In New York, diplomats said the U.N. Security Council would vote on Friday on a resolution endorsing the planned pause in fighting.
The Syrian Observatory for Human Rights, a British-based monitoring organisation, on Friday reported at least 26 air raids and artillery shelling targeting the town of Douma in rebel-held Eastern Ghouta near Damascus.
Rescue workers said five people were killed in Douma. Syrian military officials could not immediately be reached for comment.
Eastern Ghouta is regularly targeted by the Syrian army and its allies. It is a stronghold of the Jaish al-Islam rebel group, which is represented in the main opposition alliance, the High Negotiations Committee. The area has been used as a launch pad for rocket and mortar attacks on Damascus.
The HNC groups political and armed opponents of President Bashar al-Assad, and many groups fighting in northern and southern Syria have authorised it to negotiate on their behalf.
The Observatory also reported artillery bombardment by government forces and air strikes overnight in Hama province, and artillery fire by government forces in Homs province.
Fighting also resumed at dawn between rebels and government forces in the northwestern province of Latakia, where the Syrian army and its allies are trying to take back more territory from insurgents at the border with Turkey.
A spokesman for President Tayyip Erdogan said Turkey has serious worries about the plan to halt violence in Syria because of the continued fighting on the ground.
Turkey's role in the ceasefire has been complicated by its deep distrust of the Washington-backed Syrian Kurdish YPG. Ankara sees the group as a terrorist organisation and has shelled YPG positions in northern Syria in recent weeks in retaliation, it says, for cross-border fire.
Washington has supported the YPG in the fight against Islamic State in Syria.
U.S. President Barack Obama said on Thursday the United States was resolved to try to make the cessation of hostilities deal work but that "there are plenty of reasons for scepticism."
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