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At least 90 people were killed in airstrikes on the rebel-held northwestern city of Idlib and parts of Aleppo province on Saturday, a monitor said, only a day after Russia and the US agreed to a truce deal in Syria.

The Syrian Observatory for Human Rights (SOHR) said at least 58 people were killed in airstrikes on a market and several neighbourhoods in Idlib.

It was not immediately clear who carried out the raids that also wounded scores of people, some of whom have been transferred to Turkish hospitals across the border.

Sources within Syria's civil defence forces, commonly known as the White Helmets, told °®Âþµº that Saturday's airstrikes were carried out by Russian warplanes.

The Britain-based monitor could not immediately confirm how many of the casualties were civilians saying some bodies "were burned beyond recognition," but said at least 26 victims were women and children.

"People were shopping before the Eid al-Adha next week, which is why the toll is so high," said Observatory head Rami Abdel Rahman.

SOHR added that many of wounded are in severe condition and many others remain unaccounted for, which is likely to increase the death toll.

Meanwhile, multiple airstrikes on rebel-held areas in Aleppo province killed at least 30 people on Saturday, the SOHR said.

The Aleppo Media Centre, a collective of opposition activists confirmed intense airstrikes on rebel-held parts of Aleppo and nearby villages and towns, but put the death toll at 45.

Contrasting casualty figures are common in the aftermath of large attacks in Syria.

The devastating airstrikes come after marathon talks in Geneva, after which US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov said a ceasefire would begin Monday, the first day of the Muslim feast of Eid al-Adha.

If the deal is implemented, Syria's armed forces would stop airstrikes on rebel-held areas and desperately-needed aid would reach war-weary civilians.

UN investigators last week said aerial bombardment by Syrian forces and their Russian ally, namely in Idlib and Aleppo, were causing huge civilian casualties and destroying vital infrastructure.

Since spring 2015, most of Idlib province has been held by an alliance of rebels, Islamists, and jihadists known as the Army of Conquest.

The Army of Conquest - led by the Fateh al-Sham Front, which changed its name from al-Nusra Front after renouncing ties with al-Qaeda - is regularly bombarded by pro-government forces.

Close to 500,000 people have been killed since Syria's conflict first erupted in March 2011, and several attempts at securing a long-lasting truce have faltered.