Dozens of people, including 14 children, have been killed by Syrian army air raids on rebel districts of the northern city of Aleppo, activists groups said Al Jazeera reported.
The groups reported on Sunday that government forces had bombarded rebel-held civillian areas, by dropping barrels filled with explosives from helicopters.
"An attack using explosive-laden barrels over the Sakhur, Ard al-Hamra and Haydariyeh districts killed 22 people, among them 14 children and an 18-year-old boy," the Britain-based Syrian Observatory for Human Rights said after the assault on eastern Aleppo began.
Civil defence volunteers told Al Jazeera they had not rested since the morning as they tried to cope with the bombardment.
"More than 10 different areas in Aleppo came under heavy bombardment," one member of the team told Al Jazeera.
"They were shelled by both explosive barrels and missiles."
The Observatory also said the number of people killed in the town of Adra, northeast of Damascus, after a rebel faction attacked on Wednesday has risen to 28.
Observatory director Rami Abdurrahman said the dead were primarily members of President Bashar Assad's minority Alawite sect, as well as a few Druse and Shia Muslims. All three sects largely support Assad in the fight against the mainly Sunni rebels.
In an interview with Al Jazeera's Darren Jordon, Andrew Tabler, a Syria analyst at the Washington Insititute for Near East Policy, said the latest attack shows the determination of the Assad regime to retake Aleppo.
"The Assad regime has the troops to retake the area," Tabler said. "The question is if they have enough troops to hold it."
Meanwhile, UN officials visited Damascus to discuss how to help Syrians survive winter and the first United Nations aid flight from Iraq to Syria took off on Sunday after being delayed for several days owing to bad weather.
Syrians suffering
Valerie Amos, who briefly visited Syria on Saturday, said the situation was desperate.
"I'm extremely concerned with what is happening as a result of winter, the impact that it is having on Syrians not just here but in neighbouring countries," Amos said.
"I saw the foreign minister and the deputy foreign minister. We were able to talk about some of the improvements that we have been able to see in terms of our co-operation."
The UN aid plane was expected to arrive in Qamishli in north-eastern Syria about 1300GMT, according to Abeer Etefa, senior Middle East spokeswoman for the UN World Food Programme.
"Over the next few days, we will be sending to Qamishli... 400 tonnes of food," Etefa told AFP news agency.
Laurent Fabius, foreign minister of France, spoke of his concern for the Syrian people, as he said he doubted whether a peace conference planned for Geneva next month would go ahead.
He warned that a failure to hold the meeting could mean more suffering for Syrians.