Syrian troops fired tank shells and artillery Sunday at neighbourhoods in the city of Aleppo as rebels tried to repel the assault and defend their strongholds in the country’s economic hub.

The bombardment was part of a government counter-offensive to retake control of districts that had fallen into rebel hands last week.

As the regime vowed Sunday to defeat the rebels, Syria's Foreign Minister Walid al-Moallem accused regional powerhouses Saudi Arabia, Qatar and Turkey of supporting a plot hatched by Israel to destroy Syria.

The three countries have all been backing rebels trying to overthrow President Bashar Assad.

Iran, Syria's only remaining ally in the Middle East, has provided Assad's government with military and political backing for years and has continued its strong support for the regime since the uprising began.

Syria's state-run news agency, quoting government officials, said troops will continue to pursue what they call terrorists until the city of three million is purged of armed groups.

Mataz Suheil of the Syrian Observatory for Human Rights told CTV News Channel there had been intense fighting in most neighbourhoods of Aleppo since the early hours of Sunday.

“It’s been very brutal on the ground. It’s no man’s land basically,” Suheil said in an interview from London, explaining that government troops were bombarding neighbourhoods known to be rebel strongholds.

"These are vast districts of the economic capital of Syria," he said.

The rebels are estimated to be in control of up to half of Aleppo, Syria’s largest city. And the opposition claims it is holding its ground, even shooting down an attack helicopter on Saturday.

The battle for the former bastion of support for Assad is critical for both his regime and the opposition.

Capturing Aleppo, “will be very critical for opponents who want to create a northern territory where they can establish a base," Suheil said.

If the rebels were to capture the city, the regime would fall, said Leonard Spector, Deputy Director of the Monterey Institute of International Studies in an interview with CTV News Channel.

But Spector said he does not think the rebels will be victorious in Aleppo.

He said that given the scale of the regime’s military and the amount of weaponry it has, Assad’s forces will likely regain control of Aleppo and force the rebels out.

“Chances are, we will see a neighbourhood holding out for a long period of time and gradually the Assad forces will probably prevail -- they’ve got all the heavy weaponry -- and you’ll see a tactical retreat by the rebels to fight another day,” Spector said in the interview from Washington.

“Now, this won’t be over. There will be a lot of damage, a lot of death, a lot of impact on the economy. This is only one battle in what is likely to be a long struggle."

Mohammad Saeed, an Aleppo-based activist, said Sunday's shelling was some of the heaviest seen yet.

"But the rebels are still holding up well," he told the Associated Press."No ground troops have been able to enter. They are shelling from outside." He said rebels were fighting back against the attackers.

Attempts to gain control of Aleppo began a week ago and the Observatory said about 162 people, mostly civilians, have been killed in the fighting, not including soldiers. An estimated two dozen people died during Saturday’s fighting alone.

There are concerns that Assad’s desperation to maintain control of the city will intensify the violence.

“This is the concern:  We will see a massacre in Aleppo,” said Victoria Nuland, a spokesperson for the U.S. State Department.

Fears of a massacre were voiced in Canada as well.

“We think the regime is ready to kill as many people as necessary in order to regain back Aleppo,” said Khaled Sawaf of the Montreal chapter of the Syrian Canadian Council.

In a press conference on Friday, Canada’s Foreign Affairs Minister John Baird described the escalating violence by the Assad regime as “disgraceful”.

“There will be a day of accountability for what’s going on,” he said.

On Sunday, Suheil said his organization fears for the tens of thousands of Syrian nationals who have fled to refugee camps in Lebanon and Turkey. He said they are being persecuted and are wholly dependent on non-governmental organizations for their wellbeing.

“There have been protests happening in the refugee camp in south Turkey. In Lebanon, there are more than 20,000 Syrian refugees. Some are undocumented, fearing persecution by pro-government forces. The Lebanese government has stopped giving them medical aid and stopped feeding them.”

Suheil urged the UN refugee agency to do more to help Syrian refugees in Turkey and Lebanon.

According to human rights groups, up to 19,000 people have been killed since the Syrian uprising began in March 2011.

As the world’s attention focuses on Syria:

  • Jordan opened its first tent camp for Syrian refugees fleeing the ongoing violence. The camp opened on Sunday to accommodate the 142,000 Syrians seeking help from their southern neighbours. Jordanian officials said the number of refugees is growing by up to 2,000 daily.
  • Two Italian engineers who were kidnapped and held captive for eight days in Syria returned home Sunday. The men were seized by a group of masked men and the Syrian army eventually organized their release. The men were said to be in good.
  • German prosecutors said they filed criminal charges against a suspected Syrian spy. A federal spokesperson said she couldn’t release any further details until the prosecutors receive confirmation that the indictment has been received.
  • U.S. Defence Secretary Leon Panetta began a five-day trip to the Middle East on Sunday. The ongoing bloodshed in Syria is expected to be high on his agenda, as his staff said the trip is intended to reaffirm a U.S. commitment to stability in the Middle East and North Africa.
  • In his traditional Sunday Angelus prayer, Pope Benedict XVI called for an end to the violence and bloodshed in Syria. He also called on the international community to spare no effort in seeking peace and a political settlement to the conflict.