WASHINGTON/BEIRUT U.S. President Barack Obama expressed caution on Wednesday about a plan to stop fighting in Syria, while the main opposition group said it had yet to commit to the deal.
Combatants are required to say whether they will agree to the "cessation of hostilities" by noon on Friday (1000 GMT), and to halt fighting at midnight Saturday.
The United Nations hopes the planned halt in the fighting will provide a breathing space for Syrian peace talks to resume.
The last round in Geneva broke up earlier this month without progress after the Syrian government launched a Russian-backed offensive on the city of Aleppo, where more fighting was reported on Wednesday.
Obama told reporters in Washington that if some progress was made in Syria, that would lead to a political process to end the five-year-old war there.
Although U.S officials have raised the question of a political transition in Damascus, President Bashar al-Assad, backed by Russia, shows no sign of stepping aside.
The Saudi-backed HNC, which groups political and armed opponents of Assad, said on Monday it had "given its acceptance of international efforts for a cessation of hostilities".
But HNC chief negotiator Mohamad Alloush said on Wednesday that the council had not yet decided whether to commit to the deal, underlining rebel doubts over a deal they fear will not prevent Russian air strikes against them.
The deal does not include Islamic State or the Nusra Front, an al Qaeda affiliate which is widely deployed in opposition-held areas.
"How can (Russia) offer guarantees while it is part of the problem?" said Alloush, who heads the political office of the Jaish al-Islam rebel group, in an interview with the pro-opposition Orient TV station.
The Syrian government, its war effort buoyed since September by the Russian air force, has accepted the cessation of hostilities agreement announced on Monday.
Assad told Russian President Vladimir Putin on Wednesday that his government was ready to help implement the deal.
Putin and Assad, who held a telephone conversation, stressed the importance of a continued "uncompromising" fight against Islamic State, the Nusra Front and other militant groups.
U.S. Secretary of State John Kerry said he had spoken to Russian Foreign Minister Sergei Lavrov and their teams would meet in the next day or so to discuss the planned ceasefire.
"I am not here to vouch that it's absolutely going to work," Kerry said in Washington. While there had to be a diplomatic solution at some point, the question was whether the time is ripe, he added.
He asked whether Russia and Iran would work "in good faith" to bring about a political transition in Damascus.
Putin has embarked upon a round of telephone diplomacy, speaking to Assad, the Saudi king, the Iranian president and the Israeli prime minister. The Kremlin described the calls as an effort to explain the substance of the U.S.-Russia-brokered ceasefire.
The Russian Defence Ministry said it had significantly reduced the intensity of its air strikes in Syria in the past two days in areas where armed groups had expressed their readiness to join the ceasefire.
Russian state media have presented the fact that Moscow helped broker the potential ceasefire as a sign that Russia matters again on the world stage and has shrugged off what it has cast as U.S.-led efforts to isolate it over the Ukraine crisis.
Turkish President Tayyip Erdogan said he feared the ceasefire plan would do little more than benefit Assad.
Turkey has grown increasingly frustrated by the international response to the Syrian war, incensed by a Russian intervention which has tipped the balance of power in favour of Ankara's arch-enemy Assad and by U.S. support for a Kurdish militia it sees as a hostile insurgent force.
"If this is a ceasefire that is up to the mercy of Russia, which has brutally attacked the moderate opposition and aligned with Assad under the pretext of fighting Islamic State, we fear that the fire pouring over innocent people will never stop," Erdogan said in a televised speech.
The United Nations said it was ready for a huge aid effort if the fighting stops.
"We are now standing by ... waiting for the signal," a U.N. spokesman said.
The war has killed more than 250,000 people and left 4.5 million hard to reach with humanitarian aid, the U.N. says.
The United Nations carried out its first airdrop of humanitarian aid to the Syrian city of Deir al-Zor on Wednesday, delivering 21 tons of relief to civilians besieged by Islamic State.
The Syrian army and Islamic State fought fierce battles on Wednesday near Aleppo, where an attack by the jihadist group has cut the main land route to the city.
A government military source denied reports the town of Khanaser had fallen to Islamic State, although its fighters were firing on it from nearby positions.
Islamic State is escalating its assaults on government-held areas. The attacks appear to be a preemptive move, the military source said, because the militants expect to come under more pressure from the Syrian army soon.
(Additional reporting by Arshad Mohammed, Humeyra Pamuk, Tulay Karadeniz, John Davison, Ali Abdelatti, Michelle Nichols, Tom Miles and Dmitry Solovyov; writing by Tom Perry and Giles Elgood; editing by Philippa Fletcher)
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