Syrian opposition factions have gathered in Qatar to discuss forming a transitional government that could take over if the regime of President Bashar al-Assad were toppled.

Among them meeting Thursday was the prominent Syrian defector Brig.-Gen. Manaf Tlass, a commander in the powerful Republican Guard and the son of a former defence minister. Tlass defected in early July and now resides in Saudi Arabia, which backs the Syrian rebel opposition.

"I will try and help as much as I can to unite all the honourable people inside and outside Syria to put together a roadmap to get us out of this crisis, whether there is a role for me or not," Tlass said.

Opposition figures may be reluctant to accept Tlass in a leadership role, however.

"Those who recently defected from the regime must not take part in leading the transitional period," said Mahmoud Othman, an Istanbul-based member of the opposition Syrian National Council. He added that the Syrian people were paying too high a price in blood to replace Assad with someone close to him.

Meanwhile, skirmishes in Syria's commercial hub of Aleppo raged on for a sixth day Thursday, activists said, with reports that the army was shelling several neighbourhoods and launching aerial assaults as more troops were expected to arrive.

Syria's largest city had been bracing for a major showdown in anticipation of reports that dozens of tanks, as well as thousands of additional government forces, were en route to push back against a rebel offensive to seize the city of three million.

Mohammed Saeed, a local activist, told The Associated Press via Skype that clashes had been going on overnight, but that there were fears the worst was yet to come.

"Regime forces have been randomly shelling neighbourhoods and the civilians are terrified," Saeed told the news service.

"The government reinforcements have yet to arrive," he added.

According to activists, Syrian Army attack helicopters and fighter jets were deployed.

Last week, the combination of a heavy ground assault and artillery bombardments successfully quashed a rebel assault in the capital of Damascus. It appeared the troops were preparing for a similar tactic to retake Aleppo.

The stakes are high for both sides trying to wrest control of this battleground, freelance reporter Irris Makler told CBC News Thursday from Jerusalem.

"It's a vital city certainly for President Assad. It's Syria's largest city, it's Syria's wealthiest city, and so far it has been firmly behind him," she said, adding that the opposition claims to have control over half of Aleppo.

'Beginning of the end'

"If this city falls, it's a short march south towards Damascus. It is, in fact, the beginning of the end if this city falls. So Assad can't let it fall, and for that same reason, the rebels are there making this push."

U.S. State Department spokeswoman Victoria Nuland has expressed "grave concerns" about tanks and fighter jets being used in a densely populated city.

"The concern is that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," Nuland said:

The Britain-based Syrian Observatory for Human Rights said the fighting and shelling in Aleppo killed 26 people on Wednesday, including many children. It estimated that a total of 160 died throughout the country, where fighting continues in the cities of Hama, Homs, Daraa and Deir al-Zour.

Nationwide death tolls are estimated at well over 100 people a day, according to the observatory. Activists say 19,000 have been killed since the uprising began in March 2011.

While the government forces have managed to overpower the rebels, the fact it took a week to control the Damascus assault has been viewed as a sign that the opposition is growing stronger.

The rebels have been taking the fighting to areas that favour them, Makler told CBC News.

"What that means is they are in tiny areas, small alleyways the tanks can't get into," she said. "So we've seen footage over the past couple days that the rebels have released, of them running down these narrow alleys and of tanks on fire, because they're using rocket-propelled grenade launchers."

One of Assad's harshest critics is his own first cousin, Ribal al-Assad, who is living in exile in London.

Assad portrayed as puppet by cousin

Speaking to CBC Radio's Jim Brown on The Current on Thursday, the Syrian-born director of the Organization for Democracy and Freedom in Syria said Assad's inner circle of advisers were "like Mafia bosses running the country."

"Personally, I think Bashar has no power at all," he said.

"I think he's really scared of those people, that they might do something to him."

But as the opposition forces appear to ramp up their efforts, he warned that the regime will only hit back harder as the rebels try moving towards Damascus, where the country's more elite troops are based.

"These are the most trained and well-equipped divisions in the army, and those people have not entered the fight until today for the past 18 months," he said.

Ribal added that the violence suffered at the hands of the Syrian army is a reaction to an armed resistance that he also disapproves of.

"If the uprising had kept peaceful, the regime would have fallen in a couple months, but again I blame the opposition who decided to take up arms, and this helped nobody else but the regime to use excessive force, to say, 'We are fighting terrorists, we are fighting an armed insurgency,'" he said.

Canada willing to boost humanitarian aid

As the uprising against Assad intensifies, Canada's foreign affairs minister said talks were underway with Canada's allies over what to do about Syria's chemical and biological weapons, should the regime fall.

Chief among Canada's concerns is that the weapons could be used against the Syrian people, or that they could end up in the wrong hands — particularly those of Islamist extremists or al-Qaeda militants, who Middle East analysts believe may have infiltrated the rebellion.

For now, Baird said Canada will consider increasing its humanitarian aid commitment to help Syrians caught in the violence as the situation deteriorates, rather than to outright support military action in Syria.

Canada's current aid commitment of $8.5 million is the third-largest contribution by any donor country.

UNICEF launches emergency appeal

UNICEF on Wednesday launched an emergency appeal in Canada to support its Syrian emergency efforts.

The United Nations children's agency has said it needs $39 million from donor countries to support its humanitarian efforts in the region, where an estimated 1.5 million Syrians need help. So far, it says it's still $23 million short.

Meg French, UNICEF Canada’s director of international programs, told CBC News that the organization, one of the few on the ground, has been providing support for children and their families who have been displaced by the fighting.

French said many of the children are staying in schools and mosques, and being given food supplies and hygiene kits.

"Usually they have left quickly so they needed things to sustain themselves," she said, adding they have been able to reach about 190,000 people.

French said her organization is also providing psychological assistance to children.

With files from The Associated Press, The Canadian Press