The Britain-based Syrian Observatory for Human Rights monitoring group, which gathers information from a network of informants across the war-torn country, said the cars were driven by two suicide bombers from al-Qaida's Syria affiliate, the Nusra Front, which fights alongside rebel groups against government forces.
Syrian regime forces have repelled a fierce assault by opposition fighters seeking to reopen their only supply route into Aleppo city, killing at least 29 rebels, a monitor said Sunday.
The announcement came after the army forces severed the road on Thursday, following a week-long offensive.
Its capture would be a strategic prize for the Assad regime, which controls the major population centres in western Syria, apart from opposition-held areas of Aleppo, and the city of Idlib.
The Syrian army and the Russian military, which also backs president Bashar al Assad, have periodically announced a number of temporary localized truces in areas of intense fighting. Rebels hold pockets of territory elsewhere in western Syria.
Haq noted the heavy clashes along Castello road, the only road in and out of this part of the city, and said the heavy fighting over the past few days has continued to put civilians at risk.
Another aim is to ensure the security of government-controlled areas and to prevent the militant groups from blocking the main road connecting Aleppo with the Syrian coast and other provinces in central and southern Syria.
Around 200,000 people remain in the opposition-held eastern sectors of Aleppo, which has been divided between government and rebel control since shortly after fighting in the city erupted in mid-2012.
President Bashar al-Assad's allies, which include Russian Federation, say they are battling the Nusra Front in Aleppo. "There was heavy regime air cover in the night".
The attack follows a failed attempt by rebels to re-open the Castello Road, the only way out of the rebel-held east for some 300,000 people living there.
Airstrikes in Syria's Idlib and Aleppo provinces have killed 48 people as clashes escalated between government allies and rebels, despite a three-day truce being agreed to coincide with Eid al-Fitr celebrations.
The Aleppo Conquest coalition is comprised of some 31 armed groups that co-ordinate attacks against Syrian government forces, ISIS and the Kurds in Aleppo governorate.