Syrian forces clashed with rebels around Aleppo's television and radio station, activists said, and a local rebel commander said his fighters were preparing for a "strong offensive" by government forces on the city.
In the capital Damascus, troops backed by armour stormed the last opposition bastion on Friday in a drive to crush a rebel offensive that coincided with a bombing that killed four of Assad's senior security officials. The onslaught continued on Saturday as jets bombarded the city, a resident said.
Syrian forces battered Aleppo's Salaheddine district, seen as a gateway for the army into the city of 2.5 million people. The fate of the district could determine the outcome of a conflict that has already claimed some 18,000 lives.
"There is one helicopter and we're hearing two explosions every minute," a Reuters witness said.
The civil war has intensified in the past few weeks, with fighting engulfing Damascus and Aleppo, Syria's largest city, for the first time in the 17-month-old uprising against Assad family rule.
The two cities are crucial prizes for both sides in a conflict that has eluded all attempts at a diplomatic solution and risks igniting a wider conflagration.
UN member states on Friday voted overwhelmingly to condemn the Syrian government at a special session of the 193-nation General Assembly that Western diplomats said highlighted the isolation of Assad supporters Russia and China.
Western and Arab powers want Assad to step aside but Russia and China have used their Security Council vetoes to block attempts to force him out. They say outside interference is prolonging the bloodshed.
Assad's government, allied to Iran and Lebanon's armed Shi'ite Hezbollah movement, is at odds with Turkey and most of the Arab world, especially U.S.-aligned states such as Saudi Arabia and its Sunni-ruled Gulf partners.
In Salaheddine, rebels from the Free Syrian Army hid in alleyways, dodging the Syrian army's bullets and tank rounds that struck a building in the western district on Saturday.
Two fighter jets opened fire with cannon and smoke billowed from the district. A witness said: "We saw two fighter bombers that are each capable of carrying one bomb fly over the area of Salaheddine and then we heard two explosions."
Bullets zipped past residential buildings, as rebels took cover behind concrete blocks and makeshift sandbags and fired in the direction of Assad's forces.
One fighter fell to the ground after a bullet hit his chest. Fellow rebels dragged him into a car, leaving a trail of blood on the ground, to take him to the field hospital.
Explosions shook the shutters of abandoned shops and brought down a rain of concrete from buildings in Salaheddine.
A Syrian activist told Reuters the rebels had earlier 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 Syrian Army pushed from Salaheddine to al-Adhamiya where they clashed this morning with Syrian troops. But they had to retreat," the activist who identified himself as Barraa al-Halabi told Reuters.
A 19-year-old fighter called Mu'awiya al-Halabi, who was at the scene, said rebels entered the station but were driven out.
"The Syrian army sent snipers and surrounded the TV station and as soon as morning came, the army started shooting. One of our fighters was martyred and four were wounded," he said.
The Syrian Observatory for Human Rights, which said 110 people had been killed on Friday, including 88 civilians, also confirmed the clash near the station.
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. It said the army had killed dozens of fighters in Hamadaniyeh, Sukkari and Salaheddine districts and that others had surrendered.
Earlier in the day, a local rebel commander in Aleppo said he expected a Syrian army attack on rebels "within days", echoing the head of the U.N. peacekeeping department, who said there had been a "considerable build-up of military means".
"We know they are planning to attack the city using tanks and aircraft, shooting at us for three to four days and they plan to take the city," Colonel Abdel-Jabbar al-Oqaidi said.
Faced with the Syrian army's superior firepower, Oqaidi said the rebels were counting on mass defections by government soldiers once the offensive started.
"At the moment the soldiers cannot leave their bases and they are too afraid to defect. Once they are inside our city they will take off their uniforms and join us," he said.
In Damascus, a resident in the Adawi neighbourhood just north of the central Old City district, reported that jets had pounded an area of the capital on Saturday. "The bombardment has been continuous since 7 am (0400 GMT) in Tadamun district. It hasn't stopped for a moment," said the resident.
On Friday Syrian forces stormed Tadamun, the rebels' last stronghold in the city.
Syrian television said on Saturday an armed terrorist group had committed a massacre in the Damascus suburb of Yalda, near Tadamun . The television station said 20 people had been killed. It was not immediately possible to veriy the incident as Syria bars many foreign media from reporting inside the country.