Dozens of people have been killed as onslaught in Syria continues, activists say, hours after the United States and Russia hailed a breakthrough deal to put Syria's peace process back on track.

Key points: Jet strike kills 25 in city of Idlib, more killed in attacks on Aleppo

Jet strike kills 25 in city of Idlib, more killed in attacks on Aleppo Syria's government approves ceasefire deal

Syria's government approves ceasefire deal Ceasefire could lead to joint Moscow and Washington military operations

Hours after the ceasefire was agreed, warplanes bombed a marketplace in rebel-held Idlib in north-western Syria, according to rescue workers and the Syrian Observatory for Human Rights, a Britain-based monitoring group.

The observatory said 58 people, including 13 women and 13 children, were killed in raids on various neighbourhoods of Idlib city, including a market.

It said it was not immediately clear who carried out the raids, and said some bodies "were burned beyond recognition".

The rebel-jihadist alliance is most pronounced in Idlib province, the site of Saturday's deadly air strikes.

The Syrian army attacked rebel-held areas in a bid to maximise recent gains before a nationwide ceasefire comes into effect on Monday on the first day of the Muslim holiday of Eid al-Adha.

Another 12 civilians were killed in unidentified strikes on several neighbourhoods of Aleppo city, and 18 people died in bombardment of other parts of Aleppo province, the observatory said.

Insurgents said they were planning a counter offensive.

Years of fighting in the divided northern city — Syria's biggest before the civil war — have killed thousands and left residents struggling to get food and water.

The army and pro-Government militias pushed from the city's Ramousah area towards rebel pockets in the Amriyah district, both sides said.

"The fighting is flaring on all the fronts of southern Aleppo but the clashes in Amiryah are the heaviest," said Captain Abdul Salam Abdul Razak, the military spokesman of the rebel Nour al-Din al Zinki Brigades.

Recent government gains in Ramousah have reopened the main route into the Government-held west, and let forces backing President Bashar al-Assad encircle the city's rebel-held east.

The US and Russia, backing opposing sides in the war, announced a deal in the early hours of Saturday, including the nationwide ceasefire, improved aid access and joint targeting of banned militant Islamist groups.

Washington has said indiscriminate bombing of civilians by the Syrian army would have to end under an enforceable deal.

'Bring this catastrophic conflict to end': Kerry

On Saturday, US Secretary of State John Kerry called on "every Syrian stakeholder" to support the plan to "bring this catastrophic conflict to the quickest possible end through a political process".

Meanwhile, Russia's Foreign Minister Sergei Lavrov said despite continuing mistrust, the two sides had developed five documents that would enable coordination of the fight against terrorism and a revival of Syria's failed truce in an enhanced form.

John Kerry (L) and Sergei Lavrov shake hands after announcing the deal ( AFP: Fabrice Coffrini )

"This all creates the necessary conditions for resumption of the political process which has been stalling for a long time," he said.

For more than five years, the Syrian people have suffered a catastrophic series of wars that have killed or injured hundreds of thousands of people and displaced millions.

Mr Kerry said all sides in the conflict would need to adhere to the nationwide truce.

"This requires halting all attacks, including aerial bombardments, and any attempts to gain additional territory at the expense of the parties to the cessation," he said.

"It requires unimpeded and sustained humanitarian access to all of the besieged and hard-to-reach areas including Aleppo."

Syrian state news agency SANA reported, citing "informed sources" that the "Government has approved the agreement, and a cessation of hostilities will begin in Aleppo for humanitarian reasons".

The opposition High Negotiations Committee said it had yet to receive the deal's "official text", however.

Leading HNC member Bassma Kodmani said her group "cautiously welcomed" the deal but was sceptical that Damascus would comply.

"We are waiting for Russia to persuade the regime that it is necessary to commit to this agreement," she said.

Key rebel backer Turkey also supported the deal, adding that it was essential that the fighting stop and aid start flowing from day one of the ceasefire.

AFP/Reuters