Fighting raged on in southern Aleppo on Saturday, the army and rebels said, hours after the United States and Russia hailed a breakthrough deal to put Syria's peace process back on track.

The army attacked rebel-held areas, pushing to maximise recent gains before a new nationwide ceasefire was due to come into effect on Monday. 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. Violence there has scuppered past peace efforts.

The army and pro-government militias pushed from the city's the 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 U.K. based Observatory for Human Rights which tracks violence across Syria said jets believed to be either Syrian or Russian also hit rebel-held towns in the northern Aleppo countryside including Anadan and Hreitan along important insurgent supply routes.

The monitor confirmed reports by residents and activists in rebel-held eastern Aleppo who said Syrian army helicopters dropped barrel bombs on residential civilian areas in several districts.

The United States and Russia, backing opposing sides in the war, announced a deal in the early hours of Saturday including a nationwide ceasefire effective from sundown on Monday, 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.