The UN's Syria envoy said the war-battered country is facing a "crucial day" ahead of a deadline of midnight Friday for a partial truce brokered by Moscow and Washington.
President Bashar al-Assad's regime and Syria's top opposition grouping have said they will abide by the ceasefire plan, but it has been plagued by doubts after the failure of previous peace efforts.
The deal -- which excludes the Islamic State (IS) jihadist group and other extremists -- marks the biggest diplomatic push yet to help end Syria's violence.
"Tomorrow is going to be a very important, I will say a crucial day," UN envoy Staffan de Mistura told reporters at the UN's European headquarters in Geneva.
Members of the 17-nation group backing Syria's peace process are to meet in Geneva Friday to work out further details of the agreement, which is then expected to be endorsed by the UN Security Council.
There are hopes a successful ceasefire will lead to the resumption of peace talks that collapsed in Geneva earlier this month.
De Mistura said he will meet journalists around the time the ceasefire is due to take affect "to assess where we are and indicate also the information regarding the resumption of Geneva talks."
The agreement allows military action to continue against IS, which seized control of large parts of Syria and Iraq in 2014, as well as against Al-Qaeda affiliated Al-Nusra Front and other jihadist groups.
The complexities of Syria's battlefields -- where moderate rebels often fight alongside jihadist groups like Al-Nusra -- have cast serious doubt on whether the ceasefire effort will succeed.
Turkey's position towards Syrian Kurdish forces is another potential spoiler, and Turkish Prime Minister Ahmet Davutoglu said Thursday that Ankara would not be bound by the ceasefire if its national security is threatened.
"It must be known that the ceasefire is valid in Syria," Davutoglu said in televised remarks. "When it is a question of Turkey's security, then the ceasefire is not binding for us."
- Obama 'very cautious' -
Turkey has shelled Kurdish forces in northern Syria, saying the army was responding to incoming fire.
Ankara regards the main Kurdish militia in Syria, the People's Protection Units (YPG), as an offshoot of the outlawed Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency against the Turkish state.
A YPG spokesman said Thursday that Kurdish forces would respect the ceasefire but fight back if attacked.
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"We will respect it, while retaining the right to retaliate... if we are attacked," Redur Xelil said on his Facebook page.
Russia and the United States are on opposing sides of the conflict, with Moscow backing Assad and Washington supporting the opposition, but the two powers have been making a concerted push for the ceasefire to be respected.
Russian President Vladimir Putin has promised to do "whatever is necessary" to have the ceasefire implemented, although US President Barack Obama on Wednesday sounded a note of caution.
"We are very cautious about raising expectations on this," Obama said.
"If, over the next several weeks we can see some lessening of the violence... then that provides a basis for a longer-term ceasefire... and allows us to move forward to a political transition," he said.
Iran is also a supporter of Damascus, and US Secretary of State John Kerry said Thursday that Tehran had withdrawn a "significant number" of its elite Revolutionary Guards troops from Syria.
"Their presence is actually reduced in Syria," Kerry told US lawmakers.
- 'High hopes' for aid -
The United Nations has managed to boost aid ahead of the ceasefire deadline and expressed optimism on Thursday of more deliveries.
Jan Egeland, a special advisor to De Mistura, said Thursday that more than 180 trucks filled with aid had reached six areas under siege from different sides in the past two weeks.
They have brought assistance to just under a quarter of the 480,000 people estimated to be living in 17 besieged places across Syria.
Egeland said permission had been requested to bring aid to besieged parts of Aleppo, Homs and Eastern Ghouta, all hotspots in the country's conflict.
"We have high hopes that we will be able to get through to these places," he said.
On Wednesday, the UN World Food Programme (WFP) carried out its first humanitarian airdrop in Syria to try to help civilians stuck in the city of Deir Ezzor, but Egeland acknowledged the attempt had run into "problems".
The WFP said that of 21 food pallets dropped, 10 remained unaccounted for, four were destroyed and the remaining seven landed in areas where they could not be accessed.