Troops fired from helicopter gunships on several neighbourhoods of Syria's second city Aleppo on Friday, as the army faced off against rebel fighters, activists told AFP.

The Syrian Observatory for Human Rights said the army was using helicopter gunships in the southwest of the city, in the Salaheddin, Bustan al-Qasr, Sukari, Al-Mashhad and Al-Azamiya neighbourhoods.

Troops also clashed with rebels in the central Jamiliya district, adjacent to Aleppo's historic old quarter, as well as in Mahatat Baghdad and in Saadallah al-Jabiri Square.

The Observatory reported four people killed in the city on Friday morning -- three killed in shelling of the southern Fardoss district and one shot dead in the Maysaloon neighbourhood.

In Salaheddin, the scene of fierce fighting in recent days, hundreds of rebels were bracing for a threatened offensive by loyalist troops in Syria's commercial capital, which has been engulfed in violence since July 20.

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

On Thursday, violence nationwide killed 164 people -- 84 civilians, 43 soldiers and seven rebel fighters, according to the Observatory's figures.