BEIRUT: Rebels overran two police stations in Aleppo on Tuesday killing at least 40 policemen, a watchdog said, as fighting for control of Syria's commercial capital raged into a fourth day.

“Hundreds of rebels attacked the police stations in Salhin and Bab al-Nayrab (neighbourhoods) and at least 40 policemen were killed during the fighting, which lasted for hours” said Rami Abdel Rahman, director of the Britain-based Syrian Observatory for Human Rights.

The police chief was among those killed at the Salhin station in the south of the city, while three vehicles were destroyed, he added.

Pitched battles broke out before dawn in other parts of the northern city as rebels attacked key military targets, including the air force intelligence branch in Zahraa district, according to the Observatory.

Rebels armed with rocket propelled grenades later attacked Aleppo's main military court as well as a police station and a branch of the ruling Baath Party in Salhin district.

The watchdog added that the neighbourhoods of Salaheddin, Marjeh, Firdoss, Al-Mashhad, Sakhur, Al-Shaar and Ansari were shelled through the night by government troops, while the army and rebels clashed at dawn in Al-Meesr and Al-Adaa.

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 fierce fighting in Aleppo shows how crucial this city is for a regime that does not want a Benghazi in Syria,” said Abdel Rahman, referring to the coastal city secured by Libyan rebels as a base in their fight to bring down strongman Moamer Qadhafi.

He noted that gunmen from loyalist Arab tribes in Aleppo had joined the fray and were fighting alongside the army.

“Gunmen from the al-Berri family and other Arab tribes of Aleppo entered the fighting on Tuesday on the side of the regime, which wants to bring the country into a civil war,” said Abdel Rahman.

“All of this links back to calls by Syrian media and talking heads on some Lebanese satellite stations that loyal Syrian citizens should take up arms and fight with the regime troops,” he added.

Rebels on Monday seized the strategic Anadan checkpoint, some five kilometres (3.8 miles) northwest of Aleppo, securing a direct route to the Turkish border.

“During the next few hours, the impact of rebel control over this checkpoint will be proven by the amount of supplies brought to Aleppo,” said Abdel Rahman.