Syrian forces have clashed with rebels around Aleppo's television and radio station, activists say, while local rebel forces are preparing for a "strong offensive" by government forces on the country's largest city.
A Syrian activist said on Saturday that the rebels had sought to extend their area of control from the Salaheddine district, where the most intense fighting has been focused, northwards to the area around the television and radio station.
"The Free Syria Army pushed from Salaheddine to al-Adhamiya where they clashed this morning with Syrian troops. But they had to retreat," activist Barraa al-Halabi said.
The Syrian Observatory for Human Rights, a UK-based anti-government rights group, said: "Rebel forces planted explosives [at the TV station], and regime forces shelled the area before the rebels withdrew."
A Syrian army helicopter fired machinegun rounds and troops shelled rebel positions, a Reuters witness said, as they tried to break through the opposition fighter's frontline in a battleground district in the country's largest city.
There was also fierce fighting in the rebel-held quarters of Salaheddin and Seif al-Dawla, the Observatory said, as troops fought to retake the city two weeks after rebels moved in and claimed to have captured half of it.
Al Jazeera's Ahmad Zeidan, reporting from Aleppo, said the Free Syrian Army is gaining ground, despite intensifying attacks from Syrian forces.
"I would say last night has been the heaviest in terms of bombardment faced in Aleppo," he said on Saturday morning, adding there was prominent use of artillery and the use of helicopter gunships.
The SOHR said that the terrestrial signal for Syrian television in Aleppo had been cut off.
A 19-year-old fighter called Mu'awiya al-Halabi, who was at the scene said Syrian snipers surrounded the station and targeted the rebels.
"I would say last night has been the heaviest in terms of bombardment faced in Aleppo. There was prominent use of artillery and the use of helicopter gunships."
-Ahmad Zeidan, Al Jazeera correspondent in Aleppo
Syrian television said a large number of "terrorists", the term it uses for the rebels, were killed and wounded after they tried to storm the television and radio station in Aleppo.
Armour-backed Syrian troops stormed the last opposition bastion in the capital, Damascus, on Friday in a drive to crush a rebel offensive that coincided with a bomb attack that killed four of President Bashar al-Assad's top security officials last month.
Heavy explosions shook the Syrian capital Saturday and helicopters circled overhead as rebels appeared to be renewing their offensive in the city, witnesses and activists said.
"We heard heavy bombing since dawn," a witness in Damascus said, asking not to be named out of fear for his personal safety. "Helicopters are in the sky."
Syria's civil war has intensified in the last few weeks, with fighting engulfing Damascus and Aleppo for the first time in the 17-month uprising against Assad family rule.
The two cities are crucial prizes for both sides in an increasingly brutal struggle that has eluded all attempts at a diplomatic solution and risks igniting a wider conflagration.
The violence has killed at least 13 people across the country on Saturday.
Television host executed
Meanwhile, Mohammed al-Saeed, a Syrian television presenter who was kidnapped from his Damascus home in mid-July, has been executed, said the SOHR.
"The television presenter, a well-known figure on state TV, has been executed, and the al-Nusra Front has claimed responsibility for the killing," the Observatory's Rami Abdel Rahman said.
Al-Nusra, a little-known Islamist armed group, claimed the kidnapping and execution of Saeed in a statement on Friday.
"The heroes of western Ghouta [in Damascus province] imprisoned the shabih [pro-regime militia] presenter on July 19," said al-Nusra. "He was then killed after he had been interrogated."
Posted on a forum featuring the al-Qaeda flag, al-Nusra's statement showed a photograph of Saeed looking frightened, with his back against a wall in an unknown location.
"May this be a lesson to all those who support the regime," it said.
Maan Saleh, the director of state television, said there was "no material proof" of the killing.
Last month, international media rights watchdog Reporters Without Borders issued a statement on Saeed's kidnapping, and urged his captors to release him.
"News media and journalists, both professional and citizen journalists, should not be targeted by any of the parties in a war or civil war," it said at the time.
The Free Syrian Army has launched increasingly bold attacks in the country's two main cities and established strongholds in relatively central neighbourhoods in recent days.
In-depth coverage of escalating violence across Syria
The FSA claimed to have consolidated most of its control in the east of Aleppo, while also maintaining a grip on central neighbourhoods including Salaheddin and Bab al-Hadid.
Internet and telephone networks in Aleppo were mostly cut for the fourth day on Saturday, hampering attempts by rebels to co-ordinate and forcing them to use couriers to deliver orders. Soldiers were launching rockets at fighters from an infantry school north of Aleppo, Reuters reported.
Areas around the city are divided, with some villages loyal to Assad and others favouring the opposition.
Meanwhile, Ban Ki-moon, the UN chief, said "acts of brutality" reported in Aleppo could be crimes against humanity. His comments came before the UN General Assembly adopted a resolution deploring the Security Council for lack of action.
The UN chief has warned world powers they must overcome their rivalries to put an end to a "proxy war" in Syria.