Syrian government tanks backed by attack helicopters swept into Aleppo today as the regime launched an assault to regain control of the city.
The high-stakes battle for Aleppo, a commercial hub and the country's largest city, has raised fears among activists and the international community that a new massacre could be looming.
Even Syria's long-time ally, Russia, added to the chorus of alarm today, saying a "tragedy" was imminent in Aleppo. But Russia's foreign minister said it was unrealistic to expect the Syrian army to stand by while rebels were trying to take over major cities.
The latest fighting centred around the south-western district of Salaheddine, one of the first areas seized by the rebels since they began a push to control the city after being routed from the capital, Damascus.
Activists said helicopters were strafing the area and rebels faced artillery barrages and regime tanks trying to push into the area.
An Aleppo-based activist, Mohammed Saeed, said the government counter-attack had begun and rebels were fighting back in several other areas as well.
"Thanks be to God, they haven't succeeded in entering any of the neighbourhoods yet," he said.
President Bashar Assad's forces have been massing outside the city over the past few days, and Saeed said rebels from around the country have been pouring in to help defend the areas under their control.
"About 1,000 fighters have come from the Free Syrian Army from outside the province of Aleppo to help," he said, referring to the main rebel group.
State television, in a rare comment on the situation in Aleppo, reported that government forces had inflicted heavy losses on groups of terrorists, the term the regime uses for the rebels.
The pro-government daily newspaper Al-Watan called it "the mother of all battles" in a banner headline today.
The Britain-based Syrian Observatory for Human Rights said the government attack started before dawn with the sustained bombardment of several areas, followed by the movement of armoured vehicles backed by attack helicopters.
Based on reports from its network on the ground, the Observatory reported attacks in the north-eastern neighbourhood of Sakhour as well as other areas, and said the rebels had disabled a number of regime armoured vehicles.
The international community has expressed growing concern that there could be major bloodshed if Syrian troops retake Aleppo. But 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.
"The regime's destruction of its own city shows the level of oppression that has been reached in Syria," said Turkish Foreign Minister Ahmed Davutoglu from the central city of Konya. "We will do our best to stop this oppression."
Russian Foreign Minister Sergey Lavrov stood by the regime.
"Now the city of Aleppo is occupied by the armed opposition; another tragedy is imminent there," he said. "How can it be hoped that in such a situation the government will simply give in, say 'Okay, I wasn't right, overthrow me, change the regime - it's simply unrealistic."