A temporary truce brokered by the US and Russia took effect in Syria with fighting halting in key battlegrounds in the west of the country.

The 'cessation of hostilities' is not a ceasefire but it marks the first serious halt in Syria's five-year war, reached under an international agreement.

The temporary breather in the war, which has killed a quarter of a million Syrians so far, will allow aid agencies to reach people trapped in the conflict and those in dire need of humanitarian aid.

The official Syrian army and a host of insurgent groups fighting to overthrow President Bashar al-Assad had agreed to hold fire after midnight on Friday-Saturday.

"In Damascus and its countryside... for the first time in years, calm prevails," Britain-based Syrian Observatory for Human Rights director Rami Abdulrahman said, according to Reuters.

"In Latakia, calm, and at the Hmeimim air base there is no plane activity," he added, referring to Russian air strikes.

Russia, which backs President Assad, had intensified air strikes on rebel strongholds in the final few hours before the truce came into effect.

The UN reported there were 'incident's in Damascus and Deraa immediately after the midnight deadline but the situation calmed down later on.

The Russia-US truce draft was backed by the United Nations, which unanimously asked all parties to the conflict to impose a halt on fighting.

The UN resolution asked all sides to the conflict to "use their influence with the parties to the cessation of hostilities to ensure fulfilment of those commitments".

The UN will resume peace talks on 7 March if the truce "largely holds", special envoy Staffan de Mistura said.

Intensified attacks

The truce is backed by the Syrian army and around a 100 rebel factions, but not the Islamic state and its affiliates like the Nusra Front. The Front has, in fact, called for intensified attacks against the Syrian army.

The Syrian branch of al-Qaeda has also called for an escalation of attacks against President Assad's forces.

Clashes are also taking place between the US-backed Kurdish YPG and Islamic State in Raqqa province.