ALEPPO: A ceasefire brokered by Russia and the United States took effect in Syria at sundown Monday, despite scepticism over how long the truce in the five-year conflict would hold.

The initial 48-hour truce came into force at 1600 GMT across Syria except in areas held by militants like the militant Islamic State (IS) group.

Syria's armed forces immediately announced a seven-day “freeze” on military operations, lasting to midnight next Sunday.

But opposition forces had yet to formally sign on and the deal's fragility, which was underscored just hours before sundown when President Bashar al-Assad vowed to retake the whole country from “terrorists”.

The agreement, announced Friday after marathon talks between Russia and the United States, has been billed as the best chance yet to halt the bloodshed in Syria's insurgency which has raged since March 2011.

As well as bringing a temporary end to the fighting, it aims to provide crucial aid to hundreds of thousands of desperate civilians.

AFP correspondents in Syria's devastated second city Aleppo, divided between a militant-held east and government-controlled west since mid-2012, said fighting appeared to have stopped as the ceasefire took effect.

A final rocket was fired from the east into government areas just five minutes before 7:00pm local time, while rebel neighbourhoods had not been hit by bombardments for about two hours, they said.

“I was checking the time all day, waiting for it to turn 7:00,” said Khaled al-Muraweh, a 38-year-old shopkeeper in the Furqan district of western Aleppo.

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“I hope the ceasefire holds so I can see my brother who lives in the opposition-held part of the city.”

'We aren't very hopeful'

The Syrian Observatory for Human Rights said it was “quiet” on nearly all fronts across the country.

Thirteen civilians were killed in unidentified strikes in Idlib province Monday afternoon, the Observatory said.

Bombardment rocked the central town of Talbisseh all day, an activist there said, finally quieting down as the truce came into effect.

“For the past half hour, we haven't heard anything, but we aren't very hopeful... For Eid, I'm just planning on staying alive.”

Senior Russian military official Sergei Rudskoi said the “cessation of hostilities is being resumed across all the territory of Syria”.

But Russia would “continue to carry out strikes against terrorist targets”, he said.

Under the agreement, fighting will halt in areas not held by militants and aid deliveries to besieged areas will begin, with government and rebel forces ensuring unimpeded humanitarian access to Aleppo in particular.

In August, militants broke a weeks-long army encirclement of the militant-held east Aleppo, but government forces restored the blockade on September 8.

The ceasefire will be renewed every 48 hours and, if it holds for a week, Moscow and Washington will begin unprecedented joint targeting of militant forces.

Militants demand 'guarantees'

World powers have thrown their weight behind this new deal after several rounds of peace efforts failed to end the conflict that has killed more than 290,000 people.

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But Syria's militants are deeply sceptical and have asked for guarantees before endorsing the deal.

“We are asking for guarantees especially from the United States, which is a party to the agreement,” Salem al-Muslet from the High Negotiations Committee, the main opposition umbrella group, told AFP.

“We fear that Russia will classify all the Free Syrian Army ('moderate' factions) as terrorists,” as it was unclear how the deal defined “terrorist groups”, he said.

Militant groups on Sunday sent a letter to Washington saying they would “deal positively with the idea of the ceasefire” but listed several “concerns” and stopped short of a full endorsement.

“The clauses of the agreement that have been shared with us do not include any clear guarantees or monitoring mechanisms... or repercussions if there are truce violations,” they said.

Ahmad al-Saoud, who heads the US-backed Division 13 rebel group which signed the letter, said they had received no response.

A crucial part of the deal calls for 'moderate rebels' to distance themselves from the militant Fateh al-Sham Front -- previously known as Al-Nusra Front and Al Qaeda's branch in Syria -- before joint US-Russian operations begin.

But Fateh al-Sham cooperates closely with many of Syria's 'moderate rebels', including the powerful Ahrar al-Sham faction, which on Sunday issued a scathing condemnation of the Russian-US deal.

Russia's deputy foreign minister, Mikhail Bogdanov, told state-run media that UN special envoy Staffan de Mistura could be inviting parties to new peace talks “at the very beginning of October”.

Syria's government and its allies including Iran and Lebanon's Hezbollah movement have backed the truce.

But on Monday, Assad made clear he was intent on recapturing all of Syria.

“The Syrian state is determined to recover every area from the terrorists,” he said as he toured Daraya, a former rebel stronghold that surrendered last month after a four-year government encirclement.

“The armed forces are continuing their work, relentlessly and without hesitation, regardless of internal or external circumstances,” he said.