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 7:00 pm local time (1600 GMT) across Syria except in areas held by jihadists like the Islamic State group.

AFP correspondents in Syria's devastated second city Aleppo, divided between a rebel-held east and regime-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 7:00 pm, while rebel neighbourhoods had not been hit by bombardments for about two hours, they said.

The Syrian Observatory for Human Rights monitoring group said it was "quiet" on nearly all fronts.

"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.

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

In Aleppo's east, residents roamed the streets to celebrate the start of the Muslim holiday of Eid al-Adha.

"This was the calmest day since I got married a week ago," said Shadi Saber, 26, as he waited for a shave at the barber shop.

Syria's armed forces immediately announced a seven-day "freeze" on military operations, but opposition forces have yet to formally sign on.

The deal's fragility was underscored just hours before sundown when President Bashar al-Assad vowed to retake the whole country from "terrorists".

- 'We aren't very hopeful' -

The agreement, announced Friday after marathon talks between Russia and the US, has been billed as the best chance yet to halt Syria's five-year war, which has left 290,000 people dead.

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

Rebels broke a regime siege of the east in August but Assad loyalists 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 jihadist forces.

Many in the Pentagon are deeply uneasy about the proposed collaboration, with one defence official saying "the proof will be in the pudding."

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

But Moscow would "continue to carry out strikes against terrorist targets", he said.

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".

Just before the truce came into force, the Observatory reported three people killed in regime shelling on the rebel town of Douma near Damascus and another 13 killed in unidentified raids in Idlib province.

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

"We spent Eid in our bomb shelters and basements today," said Hassan Abu Nuh, referring to the Muslim feast.

"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."

- Rebels demand 'guarantees' -

But Syria's opposition and rebels remain deeply sceptical and have yet to endorse the deal.

Salem al-Muslet, spokesman for the High Negotiations Committee opposition umbrella group demanded "guarantees" on which rebel groups would be targeted.

Rebels 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.

Late Monday, the Istanbul-based opposition National Coalition said efforts to alleviate suffering were "a positive step," but reiterated the rebel call for "clear monitoring mechanisms" for the truce.

A crucial part of the deal calls for rebels to distance themselves from the jihadist Fateh al-Sham Front -- previously known as Al-Nusra Front -- before joint US-Russian operations begin.

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

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 siege.

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