In this Nov. 17, 2014 file photo, smoke rises from the Syrian city of Kobani, following an airstrike by the US led coalition, seen from a hilltop outside Suruc, on the Turkey-Syria border. AP File Photo
BEIRUT, Lebanon鈥擴S President Barack Obama warned Russia and Damacus that the 鈥渨orld will be watching,鈥� hours before a partial truce was due to come into force in war-torn Syria on Saturday.
Obama said 鈥渢he coming days will be critical鈥� for the ceasefire brokered by Moscow and Washington, and agreed to by both President Bashar al-Assad鈥檚 regime and Syria鈥檚 top opposition grouping.
READ: US, Russia announce Syria ceasefire from Saturday
The deal鈥攚hich excludes the Islamic State of Iraq and Syria (ISIS) jihadist group and other extremists鈥攎arks the biggest diplomatic push yet to help end Syria鈥檚 violence, but it has been plagued by doubts after the failure of previous peace efforts.
Members of the 17-nation group backing Syria鈥檚 peace process are to meet in Geneva on Friday to work out further details of the agreement.
It is then expected to be endorsed by the UN Security Council, also on Friday, diplomats said.
READ: UN Security Council set to endorse Syria ceasefire
There are hopes a successful ceasefire will lead to the resumption of peace talks that collapsed in Geneva earlier this month.
鈥淭omorrow is going to be a very important, I will say a crucial day,鈥� the UN鈥檚 Syria envoy Staffan de Mistura told reporters at the UN鈥檚 European headquarters in Geneva.
The agreement allows military action to continue against ISIS, which seized control of large parts of Syria and Iraq in 2014, as well as against the Al-Qaeda affiliated Al-Nusra Front and other jihadist groups.
Obama said he was 鈥渃ertain鈥� those groups would continue to fight, but stressed that the US-led coalition was winning the war against ISIS, citing territorial gains.
Obama said he was not 鈥渦nder any illusions鈥� about possible pitfalls, but said the ceasefire could be a 鈥減otential step in bringing about an end to the chaos.鈥�
鈥淎 lot of that is going to depend on whether the Syrian regime, Russia, and their allies live up to their commitments,鈥� Obama said.
Russian President Vladimir Putin has promised to do 鈥渨hatever is necessary鈥� to ensure the ceasefire is implemented.
Potential spoiler
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.
Obama reiterated his view Thursday that Assad should step down if a lasting peace is to be found.
Iran is also a supporter of Damascus, and US Secretary of State John Kerry said Thursday that Tehran had withdrawn a 鈥渟ignificant number鈥� of its elite Revolutionary Guards troops from Syria.
Turkey鈥檚 position towards Syrian Kurdish forces is a 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.
鈥淚t must be known that the ceasefire is valid in Syria,鈥� Davutoglu said. 鈥淲hen it is a question of Turkey鈥檚 security, then the ceasefire is not binding for us.鈥�
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鈥檚 Protection Units (YPG), as an offshoot of the outlawed Kurdistan Workers鈥� 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.
鈥楬igh hopes鈥� 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 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鈥檚 conflict.
鈥淲e have high hopes that we will be able to get through to these places,鈥� he said.