Fighting has mostly stopped across western and northern Syria under a cessation of hostilities which the United Nations called the best hope for peace since the civil war began five years ago.
- Fighting mostly stops in Syria under ceasefire plan
- Russia calls off air strikes for one day
- Rebels report some violence in north-west Syria
- Russian and Syrian government forces say they will continue to attack Islamic State and Al Nusra fighters
Under the US-Russian accord accepted by President Bashar al-Assad's Government and many of his enemies, fighting should cease so aid can reach civilians and talks can open to end a war that has killed more than 250,000 people and made 11 million homeless.
Russia, which says it intends to continue strikes against areas held by Islamist fighters that are not covered by the truce, said it would suspend all flights over Syria for the day on Saturday to ensure no wrong targets were hit by mistake.
A Syrian rebel commander said government shelling had stopped in some parts of Syria but continued elsewhere in what he described as a violation that could wreck the agreement.
The truce is the culmination of new diplomatic efforts that reflect a battlefield dramatically changed since Russia joined the war in September with air strikes to prop up Mr Assad.
Moscow's intervention effectively destroyed the hope his enemies have maintained for five years encouraged by Arab and Western states — to topple him by force.
The agreement is the first of its kind to be attempted in four years and, if it holds, would be the most successful truce of the war so far.
But there are weak spots in a fragile deal which has not been directly signed by the Syrian warring parties and is less binding than a formal ceasefire.
Importantly, it does not cover powerful jihadist groups such as Islamic State and the Al Nusra Front, Al Qaeda's branch in Syria.
"Let's pray that this works because frankly this is the best opportunity we can imagine the Syrian people has had for the last five years in order to see something better and hopefully something related to peace," UN Syria envoy Staffan de Mistura said at a midnight news conference in Geneva.
He said he expected occasional breaches of the agreement but called on the parties to show restraint and curb escalation.
Several insurgents in the western and northern part of the country said it had been mainly quiet so far.
Nevertheless, Fares Bayoush, head of the Fursan al-Haqq rebel group, which fights under the banner of the Free Syrian Army, said continuing violations could lead to the "collapse of the agreement".
"There are areas where the bombardment has stopped but there are areas where there are violations by the regime such as Kafr Zeita in Hama, via targeting with artillery, and likewise in Morek in northern Hama countryside."
Rebels report violence from Syrian government forces
In early reports of violence, a Syrian rebel group in the north-west said three of its fighters had been killed while repelling an attack from government ground forces a few hours after the plan came into effect.
Its spokesman called it a breach of the agreement; the Syrian military could not be reached immediately for comment.
Syria's state media said at least two people were killed and several wounded when a car bomb exploded at the entrance of Salamiya, a town east of Hama city and a frontline between government forces and Islamic State group.
The Islamic State group claimed responsibility for the attack in an online statement.
Another suicide bomber on a motorbike struck at the entrance to the village of Teeba soon afterwards, killing four people, state media said.
Armed groups also fired several rockets on residential areas in Syria's capital Damascus, state media said, quoting a military source.
The Syrian Kurdish YPG militia said Islamic State fighters had attacked Tal Abyad, a town near the Turkish border.
Damascus and Moscow say they will respect the agreement but continue to fight the Al Nusra Front and Islamic State.
Other rebels seen as moderates by the West say they fear this will be used to justify attacks on them.
Russia's defence ministry said it would suspend air strikes in a "green zone" — defined as those parts of Syria held by groups that have accepted the cessation — and make no flights at all on Saturday.
Sergei Rudskoi, a lieutenant-general in the Russian air force, told a news briefing that Moscow had sent the United States a list of 6,111 fighters who had agreed to the ceasefire deal and 74 populated areas which should not be bombed.
Al Nusra Front, one of Syria's most powerful Islamist rebel groups, often operates close to other groups, making it potentially difficult to prove whether strikes have targeted it.
On Friday, Al Nusra urged insurgents to intensify their attacks on Mr Assad and his allies.
Fighting raged across much of western Syria right up until the cessation came into effect but there was calm in many parts of the country shortly after midnight, the Syrian Observatory for Human Rights said.