Syrian rebels said a "barbaric and savage attack" was underway in a rebel-held Aleppo district, but denied claims by government troops to have captured the area.
Earlier Syrian forces claimed to have captured the Salaheddin neighbourhood after storming it and "annihilating" most of the insurgents.
The offensive came as Amnesty International raised concerns about the plight of civilians in Syria's commercial capital and warned both sides they would be held accountable for attacks on residential areas.
Early in the afternoon, state news agency SANA said "our brave armed forces have taken full control of the district of Salaheddin" and "inflicted heavy losses on groups of armed terrorists, killing or wounding a large number of them."
Dozens of rebels were captured, including foreigners, and others surrendered, SANA said. It said troops seized a large number of arms.
State television said the "armed forces dealt violent blows to the mercenary terrorists" in Salaheddin, "annihilating most of the terrorists."
But rebel Free Syrian Army (FSA) spokesman Colonel Abdel Jabbar al-Oqaidi said: "It is not true the regime army has seized control of the district.
"It is true that there is a barbaric and savage attack," he told AFP via Skype. "They are using all the weapons at their disposal to attack Salaheddin, including fighter jets, tanks and mortars."
He said there was fighting in many districts, but it was concentrated on Salaheddin because of the "great symbolic value for us and the army."
Hours later, rebel commander Wassel Ayub said the FSA had launched a counter-attack and retaken part of Salaheddin.
"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," he told AFP by telephone.
"We staged our counter-attack after 700 fighters arrived from the southern neighbourhood of Sukkari, Bustan al-Qasr, Shaar and Hanano" in the east, said Ayub, who heads the Nur al-Haq Brigade.
The Syrian Observatory for Human Rights likewise said the rebels were making gains.
"Clashes also raged in Aleppo's Al-Midan district as rebels tried to seize" an air force security bureau, the Britain-based watchdog said.
At least 37 people were killed in Aleppo -- 17 civilians, 10 rebels and 10 soldiers -- from a total of at least 162 nationwide, it said.
"I believe the real number is much higher but it is impossible to document the figures because of the ferocity of the clashes" in Aleppo, Observatory director Rami Abdel Rahman said.
A total of 225 people -- mostly civilians -- died in Syria on Tuesday, one of the worst days for casualties in the nearly 17-month uprising the Observatory said last week has cost more than 21,000 lives.
Amnesty showed satellite images indicating an apparent increased use of heavy weapons in the Aleppo area, and warned forces loyal to President Bashar al-Assad and rebels that attacks on civilians would not go unpunished.
"Amnesty International is sending a clear message to both sides in the fighting: any attacks against civilians will be clearly documented so that those responsible can be held accountable," Amnesty's Christoph Koettl said.
The London-based watchdog said images from Anadan, a small town near Aleppo, revealed more than 600 probable artillery impact craters from the fierce fighting over the city.
It said a July 31 image showed what seemed to be impact craters next to what appeared to be a residential housing complex in Anadan.
Amnesty said it was concerned the deployment of heavy weaponry in residential areas would lead to further human rights abuses and breaches of international law.
A group of 160 Syrians travelling in a fishing boat arrived in the southern Italian port of Crotone on Wednesday, the Italian ANSA news agency said.
The group from the conflict-torn country was made up mainly of families and included 76 men, 36 women and 48 children.
Their boat was spotted about six kilometres or 10 miles offshore by a coastguard unit that escorted them to port.
The Syrians, all apparently in good health, were taken to a reception centre.