BEIRUT, Feb 27 - A cessation of hostilities in Syria came into effect at the agreed time of midnight on Saturday (2200 GMT Friday) under a U.S.-Russian plan which warring sides in the five-year conflict have said they would commit to.

A monitoring group said shortly after midnight that across most of western Syria fighting appeared to have stopped, with isolated incidents of fire in some areas.

Damascus and its ally Russia, as well as a range of insurgent groups fighting against them, had said they would commit to the plan.

The truce does not apply to Islamic State and al Qaeda affiliate the Nusra Front, and the Syrian government and Moscow have said they will not halt combat against those militants.

The United Nations unanimously demanded late on Friday that all parties to the conflict comply with the terms of the plan.

Fighting had raged across much of western Syria right up until the agreement came into effect, the Britain-based Syrian Observatory for Human Rights said.

Shortly after midnight, there was calm in many parts of the country, it said.

"In Damascus and its countryside... for the first time in years, calm prevails," Observatory director Rami Abdulrahman said.

"In Latakia, calm, and at the Hmeimim air base there is no plane activity," he said in reference to the Latakia base where Russia's warplanes operate from.

In the northern city of Aleppo some gunfire had been heard shortly after midnight, and there were some blasts heard in northern Homs province, but it was not clear what had caused them, he said.

On Friday at least 40 government soldiers and allied fighters, and 18 insurgents were killed in battles and air strikes in Latakia province, the Observatory reported.

Also in the hours before the halt, six people died in an air raid in western Aleppo province, it said. Near Damascus, dozens of air raids hit besieged Daraya suburb. Rescue workers said at least five people were killed in Douma northeast of the capital.

U.N. appeal for peace talks

Nusra Front on Friday called for an escalation in fighting, urging insurgents to intensify their attacks in a call that added to the dangers facing the fragile agreement.

Under the measure, which has not been signed by the Syrian warring parties themselves and is less binding than a formal ceasefire, the government and its enemies were expected to stop shooting so aid can reach civilians and peace talks begin.

Aid has been delivered to some besieged areas of the country this year in a series of localised agreements, but the United Nations demands unhindered access to all Syrians in need of help.

Peace talks collapsed earlier this month before they began, and Damascus and Moscow intensified assaults in the north and northwest of the country.

Moscow's intervention in the war in September with an air campaign has helped Assad's forces and their allies recapture territory, notably in Aleppo and Latakia provinces.

Rebels have advanced elsewhere including in Hama province, but fighting has largely tipped in favour of Damascus, which is also backed by Lebanese Hezbollah and Iranian fighters.

Saudi Arabia, which supports insurgents, has said it is willing to send its forces into Syria to fight Islamic State, and Turkey, another Assad opponent, wants ground troops deployed but has denied plans for unilateral action.

Friday's U.N. resolution urged the government and opposition to resume talks, renewing a call to end a war in which most regional and world powers are involved.

U.N. Syria envoy Staffan de Mistura said he intends to reconvene talks on March 7 provided the halt in fighting largely holds.

The Syrian government has said the cessation plan could fail if foreign states supply rebels with weapons or insurgents use the truce to rearm.

The main Saudi-backed opposition alliance, which has deep reservations, said it would accept it for two weeks but feared the government and its allies would use it to attack opposition factions under the pretext that they were terrorists.

The U.S.-backed Kurdish YPG militia, which is battling Islamic State in the northeast and Turkish-backed rebel groups in the northwest, said it would abide by the plan, but reserves the right to respond if attacked.