US President Barack Obama says a proposed ceasefire due to come into effect on Saturday could be a key step towards ending Syria's war.
Obama told a meeting of members of his National Security Council in Washington on Thursday that halting air strikes was essential for the truce to be successful.
He added that ending fighting among various forces was the best route in tackling the Islamic State of Iraq and the Levant (ISIL).
"The only way to deal with ISIL in a way that defeats them in a lasting way is to end the chaos in the civil war that has engulfed Syria," Obama said.
"That's how ISIL was able to thrive in the first place."
Obama noted, however, that it remains to be seen whether the "cessation of hostilities" will hold. 
"None of us are under any illusions. There are many potential pitfalls and reasons for scepticism," he said.
Russia and the US set a deadline of midnight Damascus time (22GMT) on Friday for a cessation of hostilities between President Bashar al-Assad's regime and opposition forces.
The deal marks the biggest diplomatic push yet to end Syria's nearly five-year-old war which has killed more than 260,000 people and displaced millions.
Jim Walsh, from the Massachusetts Institute of Technology's security studies programme, said Obama appeared cautious about the success of the ceasefire in his statements.
"On the one hand he put his arms around this new ceasefire arrangement and identified with that, but he was quick to say scepticism was warranted - there will still be fighting with this ceasefire - and he tried to lower expectations," Walsh told Al Jazeera.    
Turkey on Friday also expressed scepticism over the viability of an upcoming ceasefire agreed between Syria's warring parties, as the Syrian regime and its ally Russia pressed ahead with an offensive.
"We support the ceasefire in principle. Turkey has played an active role in the making of this decision," presidential spokesman Ibrahim Kalin told reporters in Ankara.
"But considering what has happened so far ... we are seriously concerned over the future of the ceasefire because of the continuing Russian air raids and ground attacks by forces of al-Assad," he said.
The Kremlin confirmed Friday that Russian warplanes were continuing to bomb "terrorist organisations" in Syria hours before the deadline.
Meanwhile, war monitor the Syrian Observatory for Human Rights reported that forces loyal to Assad - supported by Russian air strikes - had taken back the strategic town of Khanaser from ISIL.
Rebels have been fighting to take control of the town, 50km southeast of Aleppo, with forces loyal to Assad.
The rebels have controlled the main highway to Aleppo, Syria's largest city, which government forces have been encircling in recent weeks. 
Al Jazeera Arabic correspondents also reported at least four people were killed in a series of Russian air strikes in Aleppo's northern and southwestern countryside.