'Crucial day' for Syria

"Tomorrow is going to be a very important, I will say a crucial day," de Mistura told reporters at the UN headquarters in Geneva on Thursday.

17 nations backing Syria's peace process were to meet in the Swiss city on Friday to work out further details in the ceasefire, due to start at midnight on Saturday morning. De Mistura is also set to announce a date for a new round of talks between Syria's warring parties.

Under the new ceasefire agreement, international powers would be allowed to continue their operations against the so-called "Islamic State" as well as the al-Qaeda-affiliated Nusra Front and other jihadist organizations.

Syrian President Bashar Assad said his forces and ally Russia would have to continue their strikes against the jihadists, but the truce would be off if foreign countries supplied rebels with arms or if anti-government fighters used the truce to rearm themselves. Opposition leaders initially set the period of the ceasefire for two weeks.

Meanwhile, fighting in the final days before the truce continued in Daraya, a suburb of Damascus held by anti-Assad fighters. According to the Britain-based Syrian Observatory for Human Rights, army helicopters dropped at least 30 "barrel bombs" on Daraya on Thursday.

The Observatory's Director Rami Abdulrahman, also confirmed heavy airstrikes on the northern city of Latakia on Wednesday and Thursday. He said the presence of the Nusra Front and similar groups would cause Assad's army to carry on with the fighting.

Aid for stranded civilians

The main purpose of the ceasefire was to allow aid to reach civilians, especially in areas cut off from supplies. UN Advisor Jan Egeland said that the ceasefire could nevertheless rescue the civilian population from "the abyss" and end the "black chapter "of sieges.

Egeland also confirmed that 180 UN aid trucks had reached 480,000 people in besieged areas. The agency had asked for permission to enter conflict-ridden parts of Aleppo, Homs and Eastern Ghouta. "We have high hopes that we will be able to get through to these places," he said.

The UN official also said a UN airdrop of food and medical supplies for 200,000 people in the besieged city of Deir el-Zour had run into problems. 21 food pallets were dropped, with some reaching inaccessible areas and some unaccounted for, the World Food Programme said.

Obama 'cautious' about outcome

The complex nature of the Syrian war and different interests of the participants have cast serious doubt on the ceasefire's success. Syria's Kurdish militia, YPG, told the Reuters news agency on Wednesday that it would halt its fire - but would respond if attacked. Turkey, which considers YPG an offshoot of the outlawed Kurdish Workers' Party (PKK), has said it will consider the truce broken if it were threatened by the so-called "Islamic State" or the YPG.

"It must be known that the ceasefire is valid in Syria... When it is a question of Turkey's security, then the ceasefire is not binding for us," Turkish Prime Minister Ahmet Davutoglu said.

Russia's President Vladimir Putin has promised to do "whatever is necessary," but his counterpart in Washington, Barack Obama expressed doubts.

"We are very cautious about raising expectations on this. If over the next several weeks we can see some lessening of the violence... then that provides a basis for a longer-term ceasefire," Obama said, adding that the subsequent political transition will help end the civil war.

mg/jr (Reuters, AFP)