Syrian opposition forces said a "barbaric and savage attack" was under way in an opposition-held Aleppo district, but denied claims by government troops to have captured the area.
Earlier Syrian regime forces claimed to have “captured” the Salaheddin neighborhood after storming it and "annihilating" most of the fighters.
The offensive came as Amnesty International raised concerns about the plight of civilians in Syria's commercial capital.
The 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, FSA 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 neighborhood 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 opposition forces were making gains.
"Clashes also raged in Aleppo's Al-Midan district as opposition fighters tried to seize" an air force security bureau, the Britain-based watchdog said.
At least 37 people were killed in Aleppo 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.
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 kilometers 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 center.
In a related development an A380 superjumbo plane carrying a French military medical team destined to help refugees on the Joranian-Syrian border took off from Paris on Thursday.
With around 25 medical and 25 logistics staff on board, the plane was to stop off in Istres on France's Mediterranean coast to pick up 20 tons of medical aid before flying to Amman.
President Francois Hollande's office announced the deployment on Monday saying the decision was made with the full consent of the Jordanian authorities.
PHOTO CAPION
Syrian opposition fighters load a wounded person into an ambulance during clashes with pro-regime forces in the al-Sahur district of Aleppo on August 7, 2012.
AFP