Syrian government aircraft dumped barrels packed with explosives on at least four opposition-held neighbourhoods of Aleppo on Wednesday, the fourth day of stepped-up airstrikes on the contested northern city, activists said.

The air campaign, which activists say killed more than 100 people in the first three days alone, suggests that Syrian President Bashar al-Assad's forces are trying to crush opposition in Aleppo ahead of an international peace conference scheduled for late January in Switzerland.

Aleppo, Syria's largest city and once its commercial hub, has been a major front in the country's civil war since the rebels launched an offensive there in mid-2012. Since then, bloody street battles have carved the city up into opposition- and government-controlled areas.

Wednesday's airstrikes hit at least four neighbourhoods in the city, said Aleppo-based activist Abu al-Hassan Marea. One of them exploded near the Ahmad al-Qassar school, while another landed by a student dormitory, he said via Skype.

'Now they just watch [the bombings]. If it seems to be heading in their direction, they hide — if they have time.' -  Mohammed Hussein, Aleppo-based activist

Marea and the Britain-based Syrian Observatory for Human Rights said at least two people were killed in the air raids.

Activists in Aleppo say the four-day offensive has been the most intense they've seen since the beginning of the uprising against Assad in March 2011.

The Syrian government frequently uses barrel bombs, which contain hundreds of kilograms (pounds) of explosives and cause massive damage on impact. Activists describe the bombs as "barrels of blood" because of their devastating effect.

The government air assault has overwhelmed Aleppo's already overtaxed medical facilities, the international aid group Doctors Without Borders warned Tuesday. It said hospitals are struggling to cope with the influx of casualties, and the facilities are running out of drugs and medical supplies.

In a reflection of Aleppo's grim reality, Marea said some of the residents have gotten used to the bombing. On Tuesday, some 100 metres from a bombing site, "people were buying and selling like nothing had happened," he said.

Another Aleppo-based activist, Mohammed Hussein said residents used to flee from where they thought the bombs would land. "Now they just watch. If it seems to be heading in their direction, they hide — if they have time," Hussein told the AP, also on Skype.

Syria's main Western-backed opposition group, the Syrian National Council, accused the international community on Tuesday of "failing to take any serious position that would guarantee a stop to the bloodbath" in Aleppo ahead of the peace talks.

Civil war has escalated

The country's conflict, now in its third year, appears to have escalated in recent weeks as both sides manoeuvre ahead of next month's planned peace talks and ignore calls for a ceasefire.

The U.S. and Russian-brokered peace conference between Assad's government and the Syrian opposition is scheduled to begin in January in the Swiss city of Montreux. Plans are underway to organize a one-day meeting of foreign ministers in the city ahead of the Syrian talks.

The civil war has ripped Syria apart. More than 120,000 people have been killed, and nearly nine million Syrians have been uprooted from their homes — some 40 per cent of the country's prewar population of 23 million. They include some 2.3 million who have fled to neighbouring countries, mostly Lebanon.

Underscoring the perilous conditions many refugees face, Lebanese state media and a municipal official said that a roof collapse in eastern Lebanon killed two Syrian refugee children early Wednesday.

The family of the children, aged nine and three, were renting a home in the town of Hazarta, said official Hussein Abu Hamdan.

Many of the impoverished Syrians have crowded into shacks, tents and unfinished structures that offer meagre shelter from the bitter cold, with no running water or electricity.

Meanwhile, a Russian cargo plane landed in Beirut on Wednesday with emergency aid for Syrian refugees. Moscow said it was the fifth planeload of aid from the Russian government.