Nearly 50 strikes hit rebel-held areas in and around the Syrian city of Aleppo on Sunday in some of the heaviest recent raids by Russian and Syrian government aircraft, residents and a monitoring agency said.

For their part, rebels also struck government-held areas of Aleppo in what Syrian media said was an escalation in mortar attacks on the western districts of what was the country's largest city before the war.

The Syrian Observatory for Human Rights said the Syrian government raids had targeted the main road that leads into rebel-held Aleppo as part of a campaign to cut the primary rebel route into the city and complete the encirclement of its insurgent-held areas.

Aleppo, which has been divided between rebel and government-held zones for years, has been the site of many deadly bombardments that have all but destroyed a February cease-fire agreement.

Full control of Aleppo would be a huge prize for Syrian President Bashar al-Assad, whose government has been bolstered by Russia's military involvement in the conflict in September.

In northern Syria, meanwhile, government forces backed by Russia advanced Friday toward a bastion of the Islamic State group (IS) in the town of Tabqa, east of the group's de facto Syrian capital of Raqa, the Observatory said.

The advance came after a US-backed Kurdish-Arab alliance pressed an offensive  toward the same town  from the opposite direction last week.

The alliance is also advancing toward the strategic IS-held town of Manbij.

The US military said the Syrian rebels advancing on the IS fighters in Manbij have seized more than 100 square kilometers this week.

Nearly 600,000 people are estimated to live under siege in Syria, most of them encircled by the forces of Assad, whose approval the UN says is needed to deliver aid by air.

Last month the UN said that if it did not see an improvement

 in aid access to besieged areas by June 1, it would task its food agency to carry out airdrops.

Syria on Friday agreed to partial aid deliveries to Moadamiyeh, Daraya and Douma, which are besieged by government forces, the UN office of humanitarian affairs said.

Allowing aid into areas under siege is key to the resumption of peace talks on ending the 5-year war that has killed 280,000 Syrians and displaced millions.

On Sunday, the UN was to  present a formal request to the Syrian government to approve airdrops where land access has been denied, diplomats at a closed UN Security Council meeting said.

Officials have stressed the challenges and risks of aid operations in the skies above a country at war.

UN spokesperson Stephane Dujarric said helicopters would have to be used to transport aid to 15 of 19 besieged areas that include densely populated towns.