Opposition groups claim they have pushed their assault deeper into regime-held areas of Aleppo, but the Syrian regime denies the assertion saying it repelled the offensive.
Syrian opposition groups opened a new front in Aleppo as fighting continued on the third day of a major opposition counter-attack to break the regime's siege of the opposition-held part of the city.
The fight for the divided city has intensified during the past three days, although it is not clear who is gaining ground, since both sides report conflicting claims.
The regime and opposition groups also accuse each other of carrying out chlorine gas attacks in the northern city of Aleppo where the forces are fighting for full control.
Groups fighting the Syrian regime have made progress in an ongoing operation to break a regime siege imposed on the northern city of Aleppo, said the commander of an opposition group.
"Our campaign aims to break the siege on Aleppo and our forces seek to enter regime-ruled neighborhoods in the city," opposition commander Ammar Saqar said.
He said they have seized regime strongholds in western Aleppo on the first day of the operation, which began on Friday.
"We have broken into the first defence lines of the Syrian regime and allied militias," he said. "The second phase of the operation will start soon."
Opposition and pro-regime sites said most of Sunday's fighting concentrated on the 3000 Apartments housing project in the al-Hamdaniya area. Capturing it would bring the opposition groups to within several kilometres of the heart of the regime-controlled area.
Russian planes resumed heavy bombing of the rebels' new locations in west Aleppo according to pro-regime sites.
Opposition group members say they had taken control of some buildings in the residential area as they seek to penetrate heavily populated areas under regime control.
They added that suicide bombers were deployed on the outskirts of the neighbourhood, a tactic used on Friday when opposition forces seized Dahiyet al-Assad, a cluster of villas once occupied by top army officers covering about a square kilometre on the southwest corner of the city.
The regime army said it had launched an offensive with its allied forces on several fronts in the west and east of Aleppo and recaptured the village of Minian on the western edge of the city, taken at the start of the campaign, and had ambushed opposition groups who had infiltrated some buildings in Dahiyet al-Assad.
Rebels denied that Minian had fallen.
Amid the intense fighting for Aleppo, both the opposition and regime forces reported the use of chemical gas around the city.
The first claim came from the regime-run media saying oppositions had fired shells containing chlorine gas at a residential area of the regime-held western part of the city, al-Hamdaniya.
It cited an Aleppo hospital director saying three dozen people - civilians and soldiers - had suffered suffocation in the alleged rebel gas attack, but did not report any deaths.
The opposition, on the other hand, denied that, and said regime forces had shelled rebel-held Rashideen district with chlorine.
The videos purportedly showed victims suffering from respiratory problems.
Aleppo, Syria's biggest pre-war city, has become the main stage of conflict between the regime, backed by Iran, Russia, and the oppositions including some supported by Turkey, Gulf monarchies and the United States.
The city has been divided for years between the regime-held western sector and the opposition-held east, which the regime and its allies put under siege this summer.
Medics say hundreds have been killed in the area since the regime launched a new offensive in September.
The opposition commanders called on residents of regime-held areas in western Aleppo to stay at home or in underground shelters as they continue their offensive.