Waves of troop reinforcements have been pouring into the northern city of Aleppo -- Syria's commercial capital -- and a security source told AFP the offensive feared by the rebels could come as early as Friday.

"The special forces were deployed on Wednesday and Thursday on the edges of the city, and more troops have arrived to take part in a generalised counter-offensive on Friday or Saturday," the security source said.

Early Friday, helicopter gunships strafed a string of rebel neighbourhoods in the southwest of the city. Clashes also broke out in the Jamiliya district, adjacent to Aleppo's historic old quarter, a human rights watchdog said.

Three people were killed in shelling of the southern Fardoss district and one was shot dead in the Maysaloon neighbourhood, the Britain-based Syrian Observatory for Human Rights reported.

In Salaheddin, a rebel bastion in the southwest of the city, hundreds of opposition fighters were bracing for the threatened counter-offensive by the embattled regime of President Bashar al-Assad.

An AFP photographer saw improvised barriers made up of sandbags and even a bus thrown up across the the streets, as well as makeshift clinics set up inside schools and mosques.

Four of the five roads between the city centre and the airport were under rebel control.

"The army's reinforcements have arrived in Aleppo," Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army (FSA), told AFP via Skype, adding that they were backed by some 100 tanks.

"We expect a major offensive at any time, specifically on areas across the southern belt, from east to west."

The front page of Thursday's edition of pro-government daily Al-Watan carried the banner headline: "Aleppo, the mother of all battles."

"Aleppo will be the last battle waged by the Syrian army to crush the terrorists and, after that, Syria will emerge from the crisis," it said.

Washington warned of the risks of a massacre by government troops in Aleppo, where rebel forces launched a major offensive on July 20 from their rear-bases across the nearby border with Turkey.

"This is the concern, that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," said State Department spokeswoman Victoria Nuland.

"Our hearts are with the people of Aleppo. And again, this is another desperate attempt by a regime that is going down to try to maintain control, and we are greatly concerned about what they are capable of in Aleppo."

But she stuck to the US position of providing only non-lethal assistance to the rebels who have been battling for 16 months to topple Assad.

"We do not believe that pouring more fuel on this fire is going to save lives. We are working in non-lethal ways. We are working to support the Syrian opposition," the spokeswoman told journalists.

As the fighting raged in Aleppo, a lawmaker from the city, Ikhlas Badawi, defected and flew to Turkey, the exiled opposition said.

"There were contacts for some time to ensure her a safe place," opposition Syrian National Council member Samir Nashhar told AFP.

"She arrived yesterday (Thursday) in Turkey and she will be going to Qatar, which has agreed to receive her."

She is the fourth member of parliament to have defected since the uprising broke out in March last year.

Nationwide, a total of 164 people were killed in violence on Thursday -- 84 civilians, 43 soldiers and seven rebel fighters, according to the Observatory's figures.

UN peacekeeping chief Herve Ladsous, who was in Damascus for talks with Syrian officials after the Security Council set a 30-day deadline for progress on a troubled peace plan, warned there was "no plan B."

"There is one political process for the time being, that is the six-point plan of the joint special envoy Kofi Annan," Ladsous told reporters in Damascus, referring to peace blueprint that was supposed to have begun with a ceasefire in April that never took hold.

"And as you know, and has been said time and again, there is no plan B. There is no alternative to that," Ladsous added.

"Syrians killing Syrians is something that should not continue."

On Wednesday, Ladsous confirmed that the United Nations had pulled out half of the 300 military observers it deployed to oversee Annan's peace plan after the Security Council on July 20 gave the mission a "final" 30-day extension of its mandate.

The mission had already suspended its patrols in mid-June in the face of the intensifying violence.