BEIRUT — In a withering four-day air assault, the Syrian government pummeled opposition-held neighborhoods in the northern city of Aleppo, leveling apartment buildings, flooding hospitals with casualties and killing nearly 200 people.

Rebels say the unusually intense airstrikes have prompted civilians to flee to the countryside and could portend a government ground offensive against the opposition-held half of the city, which has been divided for a year and half by grueling fighting.

Additional Photos

People inspect the rubble Tuesday following four days of Syrian government airstrikes in Aleppo, Syria. In a relentless offensive, helicopters dumped bombs packed with explosives and fuel on rebel-held neighborhoods. The Associated Press

The air campaign’s timing – five weeks ahead of an international peace conference – also suggests that Syrian President Bashar Assad could be trying to strengthen his position on the ground while exposing the opposition’s weaknesses before sitting down at the negotiating table.

The stakes are high in the battle for Aleppo, Syria’s largest city and a former commercial and industrial hub.

For the government, wresting back control of the entire city would deal a devastating blow to the rebels’ morale and throw doubt on the opposition’s long-term hold on the vast territory in northern Syria that it has captured over the past two years.

Since it began on Sunday, the government air assault has hammered more than a dozen neighborhoods in the rebel-held areas of Aleppo.

The campaign has killed at least 189 people and wounded 879, the aid organization Doctors Without Borders said in a statement Wednesday.

Many of the air raids have targeted neighborhoods that have seen infighting between moderate rebel factions and extremist al-Qaida-linked opposition groups, said the commander of the moderate Aleppo Swords brigade, who goes by the nom de guerre, Abu Thabet. He declined to give his full name for security reasons.

The airstrikes have overwhelmed Aleppo’s already strapped medical facilities, which are struggling to cope with the influx of casualties and are running out of drugs and medical supplies, Doctors Without Borders said.

The impact has been so devastating, in part, because of the government’s choice of weapon: helicopters that drop so-called barrel bombs containing hundreds of pounds of explosives and fuel, causing massive damage. Activists have dubbed the bombs “barrels of blood” because of their deadly effect.

“Civilians have been leaving the neighborhoods being hit and taking refuge either in villages or traveling to Turkey,” Abu Thabet said.

Other residents, however, have quickly adjusted. On Tuesday, just 100 yards from a bombing site, “people were buying and selling like nothing had happened,” said an Aleppo-based activist, Abu al-Hassan Marea.

In the past, the government has heavily bombarded civilian areas before launching a ground offensive, said Abu Thabet, adding that the current campaign may signal a major operation is imminent.

“I think the regime is planning for a new offensive. They want to advance on several fronts,” he said by telephone from Aleppo.

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