An unusually intense wave of government airstrikes has killed more than 100 people in rebel-held neighborhoods in the northern Syrian city of Aleppo, part of a pattern of intensifying violence taking shape ahead of peace talks next month, activists and doctors said Monday.

The assault, which began Sunday and continued into Monday, was the heaviest yet witnessed in Aleppo. It served as a reminder of the government’s continued military edge over the lightly armed rebels as the warring sides jostle for advantage in the lead-up to the negotiations.

The United Nations predicted Monday that the number of Syrian refugees fleeing the violence would double by the end of 2014 to more than 4 million and that more than 75 percent of the Syrian population will soon need food aid. The dire forecast suggested that even the United Nations, which is sponsoring the talks in Geneva, does not expect the effort to produce rapid results.

Appealing for $6.5 billion in new aid, an amount unprecedented for any single emergency, U.N. High Commissioner for Refugees António Guterres described “a terrifying situation” in Syria, where rebels have fought troops loyal to President Bashar al-Assad for well over two years.

“By the end of 2014, substantially more of the population of Syria could be displaced or in need of humanitarian help than not,” Guterres said. “This goes beyond anything we have seen in many, many years, and makes the need for a political solution all the much greater.”

Free Syrian Army fighters inspect damaged buildings at a site hit by what activists said was shelling from forces loyal to President Bashar al-Assad in Al-Shaar area in Aleppo December 16, 2013. (MOLHEM BARAKAT/REUTERS)

Most of the dead in Aleppo were civilians who were killed when crude barrel bombs — oil drums filled with a mixture of explosives and fuel — were dropped by helicopters over densely populated neighborhoods. One barrel hit a school, killing six students and four teachers. Others fell on apartment buildings, and one struck a busy intersection, among a total of 17 strikes.

Frantic residents coated in dust hunted for survivors under smoke-filled skies as parents led children past piles of rubble and bodies, according to videos of the scene posted on YouTube. The Aleppo Medical Council said 128 people were killed and more than 200 were injured.

“These were the most criminal and violent airstrikes ever,” said Ammar Zakaria, a physician whose hospital near Aleppo’s Old City treated scores of patients. Many could not be saved, he said, because the hospital lacks the appropriate medicine and supplies.

Majed Abdul-Nour, a citizen journalist with the opposition Sham news agency, described scenes of panic as the helicopters flew overhead. “The sight of a barrel falling from the sky is terrifying,” he said. “You feel like no matter where you run you will be hit.”

Assad’s forces have long used their aerial advantage to sustain the government’s reach in rebel-held areas.

The intensity of this week’s attacks seemed designed to expose rebel weaknesses, undermine support for their cause and increase pressure on them to negotiate, said Charles Lister, a visiting fellow at the Brookings Doha Center in Qatar. Rebel factions have been squabbling over who represents them, and recent snowy weather has limited their maneuverability on the ground.

“The government is showing it is able to up the ante, when the opposition is not able to do so,” Lister said.

Assad’s government has said that it will attend the Geneva talks but has no intention of handing over power in any form. The political opposition says it will attend only if Assad agrees to relinquish power. The rebels are so divided that they have been unable to articulate a coherent position.

Ahmad Ramadan contributed to this report.