Syrian forces pounded the country’s second city, Aleppo, with military helicopters on Saturday to flush out rebel forces in one of the most important battles of the 17-month-old uprising, activists said.

International concern has been mounting over what activists said could be a looming massacre as Syrian troops bombarded the city for the past week, unleashing artillery and strafing it with aircraft. Reinforcements have been pouring into the area in recent days.

With a population of about 3 million, Aleppo is Syria’s largest city and commercial hub, a key pillar of support for President Bashar Assad’s regime.

On Saturday, activists said soldiers were targeting rebel-controlled neighbourhoods to crush the opposition forces for good. The Britain-based Syrian Observatory for Human Rights said fierce fighting has erupted inside those neighbourhoods. The group cited its network of sources on the ground in Aleppo.

It has been a difficult two weeks for the Syrian government with rebel assaults first on the capital, Damascus, and then on Aleppo, as well as several high-profile defections and a bomb that killed four top security officials.

The government, however, launched a swift counter-offensive and quashed the assault on the capital with a combination of heavy weapons and house-to-house searches. Scores of people were killed. Opposition activists said they expected similar tactics in the coming days to keep Aleppo from falling into rebel hands.

The rebels are outgunned by the Syrian forces, making it difficult for them to hold any territory for long. But the rebels’ run on Damascus and Aleppo suggests they could be gaining in power and organisation.

Saudi Arabia and other nations have spoken positively of arming the rebels, though no country is known to be doing so.

Saudi King Abdullah announced a national campaign to collect money for “our brothers in Syria” on July 22, 2012, and on Saturday the country’s press agency said Saudi donations had reached more than $72 million.

On Friday, the U.N.’s outgoing chief observer in Syria said it’s just a matter of time before Mr. Assad’s regime crumbles, but that the violence of the civil war could worsen if Syria uses the full force of its military.

Maj. Gen. Robert Mood, whose three-month mission in Damascus ended last week, also said the situation in Syria is likely to remain unstable even if Mr. Assad’s government steps down.

“It’s impossible to imagine a future in Syria where the current people in power remain in power. So in that view, it’s just a matter of time before this regime collapses. And that is how it’s supposed to be,” Maj. Gen. Mood told a news conference in Oslo. That could happen, he said, if more members of the military simultaneously leave the ranks of the government to join the opposition.

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