British Prime Minister David Cameron, French President Francois Hollande and German Chancellor Angela Merkel will discuss the ceasefire in Syria with Russian President Vladimir Putin in a conference call on Friday, Downing Street said.

"Tomorrow is an opportunity for the leaders of the UK, France and Germany to come together... and make very clear to president Putin that we need this ceasefire to hold, to be a lasting one and to open the way for a real political transition," Cameron's spokeswoman told reporters on Thursday.

A U.N.-backed ceasefire deal took hold across parts of Syria on Saturday, bringing relative calm to areas where the Islamic State group and Al-Qaieda's local affiliate are not present.

Under the deal, a halt in fighting will take place in Damascus and most of its suburbs, the southern province of Daraa, and parts of Aleppo and Homs provinces, according to a government source and the Syrian Observatory for Human Rights monitoring group.

Fighting subsided across much of Syria Saturday as the first major ceasefire of the devastating, five-year war appeared to broadly hold despite sporadic breaches in parts of the battle-scarred country.

The truce, brokered by Washington and Moscow, is seen as a crucial step towards ending a conflict that has claimed 270,000 lives and displaced more than half the population.

Turkey on Friday expressed alarm 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.

Russia was set to deport three Syrian refugees on Thursday back to Damascus from a Moscow airport, rights activists said, in a move condemned as "shameful" by Amnesty International.

"One has just called... they are being held under guard, under police escort," head of the migrant rights group The Civic Assistance Committee, told AFP around 1500 GMT.

Iran has withdrawn a "significant number" of its Revolutionary Guards troops from the Syrian battlefield, U.S. Secretary of State John Kerry said Thursday.

Iran is an ally of Syria's President Bashar Assad and has sent members of the elite force to act as "advisers" to his forces and to organize militia units with volunteers from Iran, Iraq and Afghanistan.

Syria's army said Wednesday it will exclude an important rebel bastion near Damascus from a ceasefire set to begin at the weekend because rebel forces there include jihadists.

The announcement is further indication of the complexities of implementing the truce, which the government and opposition approved this week.

The United Nations on Wednesday carried out its first humanitarian airdrop in Syria to help civilians besieged by Islamic State jihadists, the U.N. aid chief said.

"Earlier this morning, a WFP plane dropped the first cargo of 21 tons of items into Deir Ezzor," Stephen O'Brien told the U.N. Security Council.

Turkey insisted on Tuesday there was an indisputable link between Syrian Kurdish fighters and last week's deadly attack in Ankara, amid growing confusion over the identity of the bomber.

After the February 18 suicide car bomb attack on a convoy of military buses in the capital that left 29 people dead, Turkish officials insisted the bomber was a Syrian Kurd working on behalf of the Syrian Kurdish Democratic Union Party (PYD).

Syria's regime agreed Tuesday to a ceasefire deal announced by the United States and Russia, but there were widespread doubts it could take effect by the weekend as hoped.

The truce agreement, announced Monday, does not apply to jihadists like the Islamic State group and al-Nusra Front, putting up major hurdles to how it can be implemented on Syria's complex battlefield.