CORRECTION: 12/09/2016 : An earlier version of this story reported that rebel group Ahrar al-Sham is seen as a "terrorist" group by both the US and Russia. That was incorrect. The US has not listed Ahrar al-Sham as a "terrorist" organisation.

A ceasefire in Syria brokered by Russia and the United States is set to begin at sundown after a weekend of government raids killed scores of civilians, but there are concerns about whether it will hold.

The tentative truce, announced after marathon talks by the Russian and US foreign ministers last week, has cautiously re-awakened hopes of ending a five-year civil war that has killed hundreds of thousands of people and forced millions from the country.

The Free Syrian Army group, a leading rebel alliance, said it would observe it from sundown on Monday as agreed, but with major reservations.

The alliance wrote to Washington on Sunday, saying that while it would "cooperate positively" with the ceasefire, it was concerned that it would benefit the government of President Bashar al-Assad.

Another leading rebel group Ahrar al-Sham, which is seen as a "terrorist group" by Russia, rejected the deal, saying it would strengthen Assad's government.

'Most on board'

State news agency SANA reported on Saturday that Assad's government had "approved the agreement" for a truce.

Lebanese Shia movement Hezbollah, which has intervened militarily in the war on behalf of Assad, announced its support.

Key Assad and Hezbollah backer Iran also welcomed the deal, though Foreign Ministry spokesman Bahram Ghasemi cautioned that its success would rely on creating "a comprehensive monitoring mechanism, in particular the control of borders in order to stop the dispatch of fresh terrorists" to Syria.

The deal does not include Jabhat Fateh al-Sham, formerly an al-Qaeda affiliate known as al-Nusra Front, and the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).