President Bashar al-Assad's forces are conducting coordinated ground and air attacks on Aleppo, Syria's largest city, prompting fears that a massacre maybe under way. Scores of rebels belonging to the Free Syrian Army (FSA) have been killed or badly wounded. Several casualties have been reported across the city as bombardment in the rebel-controlled districts has escalated.
Hovering helicopters and tanks are shelling the region. Western nations, the UN and Syria's neighbouring countries had earlier expressed concern that the Syrian regime was planning a massacre in Aleppo.
Forces loyal to Assad are reported to have moved tanks into the south-western districts of the city.
"Helicopters are participating in clashes at the entrance of Salaheddine district and bombarding it. There are also violent clashes at the entrances to Sakhour district," Britain-based Syrian Observatory for Human Rights told Reuters.
Salaheddine is the central district in Aleppo. Skirmishes were also reported in several other parts of the city.
An unverified video has emerged showing the intensity of the Syrian forces' shelling.
In an orchestrated move, military aircraft were seen flying at a low altitude over the region joined by ground forces and tanks closing in on the rebels.
The armed opposition groups claim that they have retaliated by destroying several military tanks.
The Syrian regime has been mustering heavy weapons and troops in Aleppo over the last few days, in a bid to crush the rebels. Both sides feel capturing Aleppo will give them a strategic advantage.
According to reports, more than 160 people were killed in the commercial capital on 27 July.
Earlier, Turkish Prime Minister Recep Tayyip Erdogan said it was not possible to remain mute spectators to the onslaught taking place in Syria's largest city.
"The most important (thing), which we have seen arise more recently, is the situation in Syria where what is happening is very important and very dangerous. There is a regime there that kills and massacres its own people," Erdogan said after meeting Prime Minister David Cameron in London.