Amman: 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 monitor said.
For their part, rebels hit government-held areas of Aleppo in what Syrian media said was an escalation in mortar attacks on the western districts.
Aleppo, the country's largest city before the war, has been divided for years between rebel and government-held zones.
It has seen many deadly bombardments that have all but destroyed a February ceasefire agreement.
Full control of Aleppo would be a huge prize for President Bashar Al Assad.
Russia's military intervention since September has helped to bolster Assad's government.
State media said missiles fired on Sunday on Hamadaniyah, Midan and other neighbourhoods by insurgents killed at least 20 people, in the second day of intense shelling of government-held areas.
The death toll over the whole weekend was at least 44.
Syria issued a toughly worded statement denouncing regional countries, holding them responsible for the latest surge in rebel-attacks and accusing them of wrecking any effort to reach a UN-backed political settlement.
Damascus says that alongside several major Western countries, these regional countries finance and train extremist rebels seeking to topple Assad's government.
In the rebel-held eastern sector of the city, the Syrian Observatory for Human Rights said dozens of barrel bombs - oil drums or cylinders packed with explosives and shrapnel- were dropped by helicopter on densely populated districts.
A civil defence worker said at least 32 people were killed in the rebel-held parts of the city, with 18 bodies pulled from flattened buildings in the Qatrji neighbourhood, the worst hit.
"This week-long campaign of bombing is very intense and day by day it's getting worse.
.it is the worst we have seen in a while," said Bebars Mishal, a civil defence official in rebel-held Aleppo.
The aerial raids on Sunday came in the wake of Friday's strikes on civilian areas that residents said were the most intense in over a month.
The monitor also said the Syrian government raids had targeted the main Castello road that leads into rebel-held Aleppo as part of a campaign to complete the encirclement of the city's insurgent-held areas.
A Russian defence ministry statement on Sunday accused militant Syrian extremist groups of firing mortars on the mainly Kurdish-populated Sheikh Maqsood neighbourhood in Aleppo that overlooks the Castello road.
The monitor said 13 people, including six children, were killed on Saturday in the Kurdish-run area by insurgent mortars.
Rebels accuse the powerful Kurdish YPG of working with the Syrian army to cut the main artery by intensifying their ground attacks on the highway.
The Russians had on Saturday accused militants from radical Islamic groups of bringing at least 1,000 fighters into an area in the southern Aleppo countryside.
The militants have consolidated gains since Friday in the area around the strategic town of Khan Touman, rebels say.
The Nusra Front spearheaded an attack on Khan Touman last month, delivering one of the biggest battlefield setbacks yet to a coalition of foreign Shi'ite fighters supporting Syrian government forces..
Rebels say Russian jets on Sunday pounded insurgent positions in the area to prevent them from advancing towards the nearby town of Hader.
Also US-backed forces on Sunday engaged with IS fighters in an offensive that began last Tuesday against IS-held areas in Aleppo province, beginning with the Manbij area where they continued to seize more villages, according to Kurdish sources and the monitor.