Syrian rebels in Aleppo have begun stockpiling ammunition and medical supplies as government forces prepare outside the city for a major battle.
Artillery and helicopter gunships have resumed attacking rebel targets and 14 people have been killed, activists say.
Troops and tanks are said to be ringing the city and reinforcements are reportedly on their way to join them.
In Damascus, activists said the army had pounded the capital's last rebel-held areas and 20 civilians had died.
Five children and four women were killed, according to London-based pro-opposition group Syrian Observatory for Human Rights, when shells hit the Yalda area. Helicopters also attacked the nearby district of Hajar al-Aswad.
The government has said its forces are trying to dislodge the "remnants of mercenary terrorist groups".
Checkpoints and snipers
In Syria's commercial hub of Aleppo, rebels have set up checkpoints and sniper positions and claim to be in control of half of the city.
A Syrian security source was quoted by AFP news agency as saying that special forces had begun arriving on the edges of Aleppo in readiness for a "generalised counter-offensive on Friday or Saturday".
A similar account has emerged from the Syrian Observatory for Human Rights, which talks of reinforcements arriving from the main Damascus to Aleppo road to the south.
Columns of troops and tanks are also thought to be travelling from the city of Hama, and from the border posts with Turkey in Idlib province.
Aleppo is the country's commercial capital, and key to the battle to control the north of the country.
Street battles have been reported throughout the city for days, as rebels try to hold on to neighbourhoods captured in an offensive that began at the weekend.
Although it was relatively quiet on Thursday, activists said artillery fire could be heard in the Salah al-Din neighbourhood in the southwest of the city and clashes in Tariq al-Bab in the east had led to two rebels being killed.
Foreign journalists operate under heavy restrictions in Syria so claims made by either side are difficult to verify.
The BBC's Ian Pannell near the city says thousands of people are leaving as fears grow that an intense battle is looming.
He says the rebels are reinforcing their ranks with more fighters, medical supplies, and ammunition such as Kalashnikov rifles.
Talal al-Mayhani, an activist with connections in the rebel movement in Aleppo, said the battle for the city was likely to play out in a similar way to an earlier battle in the capital Damascus, when rebels took control of large parts of the city before being forced to withdraw in the face of a government offensive.
Reporter Laurent Van der Stockt from French newspaper Le Monde told the BBC that Free Syrian Army (FSA) commanders in Aleppo were full of confidence but were lacking weapons.
"A commander told me this morning they attacked a convoy and destroyed 10 armoured vehicles."
Luke Harding of the Guardian newspaper, 50km (30mi) west of Aleppo, told the BBC that much of the rest of Aleppo province was in rebel hands but it was an exaggeration to say they were in control of half of the city.
"It's true they're in the southern part of the city and the northeast. Tactically, they're doing guerrilla-style attacks and they can melt away quite quickly."
In Damascus, where government forces have largely repelled a sustained assault by rebels, state TV reported more clashes in the southern district of Qadam.
The report showed army troops firing intensively as they moved into heavily damaged streets.
The violence also spread to the Yarmouk Palestinian refugee camp, where activists said street battles had taken place between rebels and soldiers.
More than 16,000 people have been killed in Syria since the start of anti-regime protests in March 2011.
Repeated diplomatic attempts to stop the violence have foundered, with the UN Security Council bitterly divided.
Thousands of refugees have already sought shelter in Turkey, Lebanon, Jordan and Iraq.
And conditions throughout the country have worsened as the fighting has spread and intensified in the two main cities in recent weeks.