BEIRUT, Lebanon - Hospitals in the northern Syrian city of Aleppo are overwhelmed with casualties, an international aid group warned Tuesday, as government warplanes blasted opposition areas of the city as part of a withering three-day air assault that has killed more than 100 people.

The intensified air campaign, which one activist group in the city called unprecedented, suggests President Bashar al-Assad's government is trying to crush opposition in the contested city, Syria's largest, ahead of an international peace conference scheduled for late January in Switzerland.

Aleppo has been a major front in Syria's civil war since the rebels launched an offensive there in mid-2012, and the city has been carved into opposition- and government-held areas. On Tuesday, the main Western-backed opposition group, the Syrian National Council, accused the international community of "failing to take any serious position that would guarantee a stop to the bloodbath" ahead of the peace talks.

The Britain-based Syrian Observatory for Human Rights said air strikes Tuesday killed 15 people, including two children, in the rebel-held Shaar district.

An amateur video posted online showed the aftermath of one of the strikes: rescue workers in white hard hats pulling a man from the rubble of an apartment building. A crowd of people in the street shouted "God is greatest!" as the rescuers rushed the dust-covered man to an ambulance. The video appeared genuine and corresponded to other AP reporting.

Tuesday marked the third consecutive day that Aleppo was hit by government air strikes.

On Sunday, at least 76 people, including 28 children, died in air raids, according to the Observatory. Other opposition groups reported higher death tolls. Another round of government air strikes Monday killed at least 12, the Observatory said.

The aid group Doctors Without Borders, which provides Aleppo hospitals with medical supplies, said in a statement Tuesday that hospitals in the city had been overwhelmed by the wounded from the attacks, which have "emptied stocks of critical drugs and medical materials for lifesaving procedures."

"Repeated attacks often lead to chaos and make it more difficult to treat the wounded, thereby increasing the number of fatalities," said the group's coordinator in Syria, Aitor Zabalgogeazkoa. "The ambulances are overwhelmed because they are called to several areas at the same time."

Syria's nearly three-year-old conflict appears to have escalated in recent weeks as both sides maneuver ahead of next month's planned peace talks, and despite calls for a cease-fire.Nearly nine million Syrians have been uprooted from their homes, with 2.3 million fleeing into neighboring countries and millions searching for shelter in safer parts of Syria.