A landmark United Nations-backed ceasefire has come into effect in Syria - the first major truce in a five-year civil war that has claimed more than 270,000 lives.

Fighting raged right up until the ceasefire took force at midnight (8:00am AEST), but guns then fell silent in the Damascus suburbs and the devastated northern city of Aleppo, AFP correspondents said.

Russia and the United States, the sponsors of the ceasefire deal, have said applying it will be difficult in a country that has been torn apart by a conflict that broke out in March 2011.

US President Barack Obama has warned Damascus and its key ally Moscow that the "world will be watching".

UN envoy Staffan de Mistura said Syria peace talks which collapsed earlier this month in Geneva would resume on March 7 if the ceasefire held and more aid was delivered.

Less than an hour before the ceasefire, the UN Security Council gave its unanimous backing to a resolution drafted by the US and Russia, demanding that it be upheld.

Both President Bashar al-Assad's regime and the main opposition body have agreed to the deal - which allows fighting to continue against the Islamic State group and the Al-Qaeda affiliated Al-Nusra Front.

The agreement marks the biggest diplomatic push yet to help end Syria's violence, but has been plagued by doubts after previous peace efforts failed.

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 are expected to stop shooting so aid can reach civilians.

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

"Humanitarian deliveries must not depend on political negotiations but must be allowed to continue and increase regardless of any truce or ceasefire," International Committee of the Red Cross chief Peter Maurer said.

Fighting rages until moment of ceasefire

Clashes raged across much of western Syria ahead of the planned halt as heavy air strikes hit rebel-held areas near Damascus.

The Syrian Observatory for Human Rights, a Britain-based monitor, said suspected Russian war planes struck near Aleppo in the suburbs of Damascus ahead of the deadline.

"It's more intense than usual. It's as if they want to subdue rebels in these regions or score points before the ceasefire," Observatory head Rami Abdel Rahman said.

He also said at least 40 members of the regime forces had been killed battling rebels in northern Latakia province, Mr Assad's heartland, in the 24 hours before the ceasefire.

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Russia launched air strikes in Syria last September saying it was targeting "terrorists", but critics have accused Moscow of hitting rebel forces in support of Mr Assad.

The intensified attacks prompted Turkey, a key supporter of opposition forces, to express worries over the viability of the ceasefire.

"We are seriously concerned over the future of the ceasefire because of the continuing Russian air raids and ground attacks by forces of Assad," presidential spokesman Ibrahim Kalin told reporters in Ankara.

The complexity of Syria's battlefields — where moderate and Islamist rebel forces often fight alongside jihadist groups such as the Al-Nusra Front — has raised serious doubts about the feasibility of the ceasefire.

While about 100 mainstream rebel groups signed up for the truce, Al-Nusra Front on Friday called for an escalation in fighting.

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 US-backed Kurdish YPG militia, which is battling Islamic State in the north-east and Turkish-backed rebel groups in the north-west, said it would abide by the plan, but reserves the right to respond if attacked.

Russia and the US are on opposing sides of the conflict, with Moscow backing Assad and Washington supporting the opposition, but the two powers have been making a concerted push for the deal to be respected.

Russian President Vladimir Putin has insisted Moscow would continue targeting "terrorist groups".

"The decisive fight against them will, without doubt, be continued," he said in televised remarks.

"We understand fully and take into account that this will be a complicated, and maybe even contradictory process of reconciliation, but there is no other way."

Iran, another key Assad ally, has said it is confident the regime will abide by the agreement.

In Washington, Mr Obama put the onus firmly on the regime and Russia.

"The coming days will be critical, and the world will be watching," he said.

Reuters/AFP

