Air strikes killed at least 50 people in rebel-held areas of Idlib and Aleppo on Saturday, just hours after Russia and the US announced a deal intended to put a stop to more than five years of fighting.

Fighter jets believed to be Russian hit a crowded market in Idlib province on Saturday afternoon, killing at least 30 people, according to an Al Jazeera reporter at the scene.

New Syria ceasefire to begin at start of Eid

"A Russian warplane targeted a residential area and a market in Idlib," Al Jazeera's Adham Abu al-Husam said, as civil defence forces, firemen and paramedics worked to pull survivors from the rubble. 

"The marketplace was full of civilians shopping for the upcoming Eid holiday."

Separate air strikes on rebel-held neighbourhoods of Aleppo city and the surrounding countryside killed at least 20 people, according to local rescue workers, and 10 people were killed by rebel shelling on the government-controlled neighbourhood of Salahuddin.

The Syrian opposition on Saturday cautiously welcomed the ceasefire deal agreed by Moscow and Washington that could also see the first joint military campaign by the two powers against hardline groups in the war-torn country.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov said that the truce, reached in Geneva late Friday, would come into force on Monday, the first day of the Muslim holiday of Eid al-Adha.

READ MORE: US and Russia clinch Syria ceasefire deal

The two powers back opposing sides in the conflict, with Moscow supporting the regime of President Bashar al-Assad and Washington backing a coalition of rebels it regards as moderate.

The Assad government in Damascus announced late on Saturday that it had agreed to the ceasefire terms, Syrian state media reported.

If Russia is able to persuade Assad to respect the ceasefire for a week, Moscow and Washington will set up a joint coordination unit to bomb agreed "terrorist" targets.

'Military response'

A leading member of the High Negotiations Committee (HNC) said the opposition umbrella group cautiously welcomed the agreement, but said it required "effective enforcement mechanisms" if any truce deal is to "credible". 

Syria: HNC's Bassma Kodmani reacts to ceasefire deal

"A violation by the regime should be met with a perhaps military response. That is what makes it credible, really," said Bassma Kodmani.

"If we we have a credible line of cessation of hostilities, then we can look to moderate groups in the opposition to disassociate themselves from extremists." 

In a letter sent to Syrian rebels on Saturday, US State Department envoy Michael Ratney urged armed opposition groups to distance themselves from Jabhat Fateh al-Sham, formerly known as the Nusra Front, or there would be "severe consequences". 

The ceasefire must hold for seven consecutive days before the US and Russia will start coordinating any attacks against fighters affiliated with al-Qaeda in Syria or the Islamic State in Iraq and the Levant (ISIL, also known as ISIS), the letter said. 

Decision time for rebels 

Both Kerry and Lavrov said the complex plan is the best available chance to end fighting between forces loyal to Bashar al-Assad and mainstream rebels while still targeting hardline fighters affiliated with the al-Qaeda-linked Nusra Front and ISIL.

"Today, the United States and Russia are announcing a plan which we hope will reduce violence, ease suffering and resume movement towards a negotiated peace and a political transition in Syria," Kerry said.

Lavrov said the US and Russia agreed on zones in which joint "strikes against terrorists" would be conducted.

The vexed question of Assad's fate remains, with Western powers calling for his ouster and Russia backing him. 

HNC unveils road map for transition

Key to the deal is the delivery of desperately needed aid to civilians in rebel-held areas of Syria's second city Aleppo, which are under siege by government forces.

Russia also needs to persuade the Syrian air force to stop strikes on rebel-held areas, which have killed large numbers of civilians.

In turn, Washington must get opposition groups it backs to separate themselves from the former Nusra Front, now called Fateh al-Sham Front, which has allied itself with a range of rebels at different points in the fluid conflict.

Kodmani said the rebels would break ranks with "extremist groups", if the truce deal held.

"The moderate groups will reorganise and distance themselves from the radical groups. We will do our part," she said.

Source: Al Jazeera News And Agencies