Syrian government tanks swept into Aleppo on Saturday in an attempt to seize control of the city of three million, but after a day of fighting rebel groups said they managed to fight off the attack.

The rebels are estimated to be in control of up to half of Aleppo, Syria’s largest city and the commercial hub of the country.

According to the Britain-based Syrian Observatory for Human Rights, the government appeared to have pulled back from its ground offensive on Saturday and it was resuming its bombardment of various neighbourhoods with artillery.

Local activist Mohammed Saeed said the rebels managed to keep the regime's tanks at bay with rocket-propelled grenades.

"The army hasn't been able to take any neighbourhoods yet, there are too many from the Free Syrian Army," Saeed said, referring to the rebels.

He estimated that about 1,000 fighters had poured into the city in the past few days to take on the Syrian army, which over the last few days has surrounded the city borders.

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.

Many residents have fled the sprawling city to the nearly refugee camps in Turkey.

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

The state-controlled al-Watan newspaper celebrated the assault and called the fight in Aleppo as the “mother of all battles.”

However, fears of a massacre in the city have generated an international outcry.

The United Nations raised fears of a massacre in Aleppo on Friday as government forces unleashed artillery, tanks and helicopters in an attempt to retake the rebel-held neighbourhoods.

UN Sectary General Ban Ki-moon called on the Syrian rebels and government forces to stop the fighting for the sake of Syrian civilians.

During 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.

However, Western nations and their allies have found themselves powerless to prevent the situation from deteriorating despite a series of diplomatic efforts, including a cease-fire agreement that never took effect.

In a statement, the Arab League expressed "deep dissatisfaction for the Syrian regime's acts of oppression," particularly the use of heavy weapons against its own people. It urged Syria "to stop the cycle of killing and violence and lift the siege off the Syrian neighbourhoods under attack."

While Russia, a key ally of Syria, said it was “simply unrealistic” for the government to give up power.

"Now the city of Aleppo is occupied by the armed opposition; another tragedy is imminent there," Russian Foreign Minister Sergey Lavrovsaid on Saturday. "How can it be hoped that in such a situation the government will simply give in, say 'OK, I wasn't right, overthrow me, change the regime.’ It's simply unrealistic."

It's been a difficult two weeks for the Syrian government, with assaults on its two main cities, a bomb that killed four top security officials and a string of high-profile defections.

However, the country's military apparatus has remained intact and continues to crush the opposition's remnants in the capital of Damascus and its outskirts.

With files from The Associated Press