Syrian government forces have taken full control of strategic district Salah al-Din in the biggest city, Aleppo, after fierce fighting, state media say.
However, rebel commanders denied they had retreated from the Salah al-Din district, amid reports of a push by army tanks and armored vehicles.
Images have emerged of bodies in shattered buildings, after the bombardment of a town near Aleppo.
Opposition activists reported at least 20 deaths across Syria on Wednesday.
In another development, Jordanian officials confirmed that former Syrian Prime Minister Riad Hijab had only just crossed into Jordan – two days after his defection was announced.
Activists said he and his family had been hiding in the south of the country and that reports on Monday that he had already left Syria were aimed at throwing government forces off the trail.
State media said government forces had taken full control of Salah al-Din, killing most of the rebels there.
They also reported heavy rebel losses near the historic citadel and in another district.
But a commander from the rebel Free Syrian Army denied that it had withdrawn from Salah al-Din, and said it had launched a counter-attack following the arrival of some 700 reinforcements.
“For an hour and a half the Free Syrian Army has staged a counter-attack and reclaimed three streets out of five seized by regime forces,” Wassel Ayub told AFP news agency by phone.
Another FSA commander, Abdel Jabbar al-Oqaidi, told AFP news agency via Skype: “It is not true the regime army has seized control of the district.
“It is true that there is a barbaric and savage attack.”
A team of Reuters news agency journalists who tried to enter Salah al-Din on Wednesday found that a checkpoint manned by rebel fighters this week had disappeared.
They were told by a lone rebel fighter that the rebels had abandoned their positions. Explosions could be heard as incoming gunfire hit buildings in the area, the Reuters team said.
Journalist Martin Chulov, of the UK’s Guardian newspaper, tweeted from Aleppo that the battle for Salah al-Din would likely decide the fate of the city.
Videos circulated on Syrian social media on Wednesday show people searching the debris of buildings in a town identified as Tal Rifaat, near Aleppo. One clip shows a plane passing overhead followed by an explosion on the ground.
Photos released by Reuters show people recovering body parts from the wreckage. At least one child appears to be among the dead.
Earlier, Amnesty International said satellite images had revealed at least 600 probable artillery impact craters in Anadan, also near Aleppo.
It said any attacks against civilians would be documented so that those responsible could be held accountable.