US President Barack Obama has expressed caution about a plan to stop fighting in Syria, while the main opposition group says it has yet to commit to the deal.
Combatants are required to say whether they will agree to the "cessation of hostilities" by noon on Friday (2100 AEDT on Saturday), and to halt fighting at midnight on 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.
"We are very cautious about raising expectations on this," he said.
Although US 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 agreement, underlining rebel doubts over a deal they fear will not prevent Russian air strikes against them.
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.
US 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.
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