Syrian troops have fired tank shells and artillery at neighbourhoods in Aleppo as rebels tried to repel a government ground assault against their positions in Syria's largest city, activists said.
The bombardment was part of a government counter-offensive to retake control of districts that had fallen into rebel hands last week at the beginning of their bid to capture Aleppo, the nation's commercial hub.
Activists said the shelling was most intense in the south-western neighbourhoods of Salaheddine and parts of Saif al-Dawla, some of the first areas seized by the rebels when they started the push last week after being routed in a similar attack against the capital Damascus.
The battle for control of Aleppo, once a bastion of support for President Bashar Assad's regime, is critical for both the regime and the opposition. Its fall would be a major blow to the regime's morale and a realisation of the opposition's crucial and strategic goal to establish a stronghold in northern Syria.
The government pledged to continue fighting and said security agents were hunting down armed groups in several areas of Aleppo including Saleheddine, inflicting heavy losses upon the "terrorists" - the term authorities use to describe Syrian rebels.
The SANA news agency quoted an Aleppo official as saying that troops would continue until the city is "purged" of armed groups and peace and tranquility is restored to its residents.
"We are seeing some of the heaviest bombardment today, but the rebels are still holding up well," said Aleppo-based activist Mohammed Saeed. "No ground troops have been able to enter. They are shelling from outside." He said rebels were fighting back against the attackers.
He said around 200 fighters entered the city today to join about 1,000 fighters who had poured into the city in the past few days to take on the Syrian army, which had been massing forces around the city ahead of the bombardment.
He also said rebels have received "a new batch of weapons and ammunition," but declined to say from where.
The British-based Syrian Observatory for Human Rights also reported heavy fighting and explosions in Aleppo. It reported intense clashes in the Bab al-Hadeed, al-Zahraa and al-Arqoub neighbourhoods.