Syrian troops and rebels fought pitched battles near an intelligence headquarters in Aleppo on Tuesday, a watchdog said, as a military offensive in Syria's commercial capital raged into a fourth day.

The fighting erupted when rebels launched an assault before dawn on the powerful air force intelligence branch in Aleppo's Zahraa district, the Syrian Observatory for Human Rights said. Fighting was continuing into the day.

Also Tuesday, rebels armed with rocket propelled grenades attacked Aleppo's main military court as well as a police station and a branch of the ruling Baath Party in the city's southern Salhin district, the Britain-based Observatory said.

Meanwhile, the neighbourhoods of Firdoss, Al-Mashhad and Ansari were bombarded through the night by government troops, the watchdog said.

Fighting also flared in the southwestern district of Salaheddin, the rebels' main bastion in Aleppo, which was strafed by government helicopter gunships, according to the Syrian Revolution General Committee, a network of activists on the ground.

A security official in Damascus told AFP on Monday that the army had regained some of Salaheddin but it was facing "a very strong resistance." The rebels, however, denied that the army had advanced even "one metre" (yard).

The Observatory said violence across the country on Monday saw 93 people killed -- 41 civilians, 19 rebels and 33 soldiers.

Among Monday's death toll were at least five rebels and one civilian killed in Salaheddin, while eight bodies of unidentified men were found near the air force intelligence branch in Zahraa.

The Observatory says it cannot swiftly give an authoritative death toll for the fierce battles in Aleppo, which began when government troops launched an all-out assault against rebel-held districts in the northern city.

"In a war context, it takes more time to accurately document the death toll," Observatory director Rami Abdel Rahman told AFP.

Just outside Aleppo, rebels seized a strategic checkpoint after a 10-hour battle on Monday, an AFP correspondent witnessed.

By securing the checkpoint, some five kilometres (3.8 miles) northwest of Aleppo, the rebels now control a direct route between the Turkish border and the commercial capital.

The United Nations estimates that around 200,000 civilians have fled the fighting in Aleppo and that many more are trapped.