BEIRUT - Syria's opposition was weighing whether to take part in a truce brokered by Russia and the United States due to start on Monday, after air strikes killed dozens in rebel-held areas.

Brokered after marathon talks by the Russian and US foreign ministers, the ceasefire has been billed as the best chance yet to end Syria's five-year civil war.

Key regime ally Iran welcomed the plan on Sunday and called for "comprehensive monitoring" of the truce, particularly along Syria's volatile borders.

But even as world powers threw their support behind the agreement, scores were reported dead from a barrage of unidentified raids in two key northern cities in opposition-held territory.

At least 62 people, including 13 women and 13 children, were killed in Saturday's bombardment on Idlib city, the Syrian Observatory for Human Rights monitoring group said Sunday.

The strikes hit several areas in the rebel-held city, including a market full of shoppers preparing for the Muslim holiday of Eid al-Adha, which begins Monday.

Britain's special representative for Syria, Gareth Bayley, called the attacks "barbaric".  "Bring on the #SyriaCeasefire," he tweeted on Sunday.

Fresh bombardment also hit the battleground city of Aleppo on Sunday, the Observatory said, a day after 12 civilians were killed in unidentified raids there. No information on casualties from Sunday's bombardment was immediately available.

"We hope there will be a ceasefire so that civilians can get a break. The shelling goes on night and day, there are targeted killings, besieged cities," said Abu Abdullah, who lives in Aleppo's rebel-held east.

In a major blow to the opposition, pro-regime forces reimposed a devastating siege on the city's eastern districts last week.

State news agency SANA on Saturday reported that Syrian President Bashar al-Assad's government "approved the agreement".

Lebanese Shiite militia Hezbollah, which has intervened militarily on behalf of Assad, said late Saturday it supported the deal.

Iranian foreign ministry spokesman Bahram Ghasemi said Sunday that Tehran, a key Assad supporter, also backed the agreement.

But Ghasemi said its success relied on the creation of "a comprehensive monitoring mechanism, in particular control of borders in order to stop the dispatch of fresh terrorists" to Syria.

Syria's main opposition group the High Negotiations Committee - which brings together political dissidents with armed rebel factions - had yet to formally respond.

Leading rebel figure Mohammad Alloush on Sunday said the agreement was "still being studied".

HNC member Bassma Kodmani told AFP on Saturday that her group "cautiously welcomed" the deal but was sceptical that Damascus would comply.

Ahmad Saoud, head of the US-backed Division 13 rebel group, wrote on Twitter he was "starting to feel that the truce is a military trap to kill us even more."

And hardline Islamist group Ahrar al-Sham, which works closely with former Al-Qaeda affiliate Fateh al-Sham Front, said Sunday it was "still discussing the points of the truce."

The agreement was reached after talks between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov in Geneva.

It would see fighting and indiscriminate air attacks halt across the country, starting at sundown on Monday for a period of 48 hours, which could then be renewed.

To get aid into the battered second city of Aleppo, a "demilitarised zone" would be established around the Castello Road into the city.

If the ceasefire holds for one week, the US and Russia could start joint operations against militants from the Islamic State group and Fateh al-Sham, formerly known as Al-Nusra Front.

On the ground, residents were split on whether a truce could hold, saying they were exhausted after several rounds of failed ceasefires, most recently in February.

"The Syrian people - whether in areas controlled by the regime or the opposition - have lost faith in both sides," said Safwan Badawi in east Aleppo.

Abu Hasan, who works with a local civil society group, laughed off the ceasefire plan as "a joke".

Pro-government Syrian newspaper Al-Watan's front page on Sunday said the deal would pave the way for renewed peace talks in Geneva.

"More importantly, Washington has finally agreed to joint military action" against militant groups, it wrote, as Kerry had been "forced to present a number of compromises to the Russian negotiator."

Several attempts at peace negotiations have faltered since Syria's war erupted, with talks earlier this year in Geneva fizzling after the opposition walked away in protest at the humanitarian situation.

Damascus resident Taher Ibrahim told AFP he did not expect the new deal to play out any differently than February's tattered truce.

"Nobody among the Syrian population accepts this agreement... (the opposition) are all the same and none of them will commit to this truce," he said.