ALEPPO - Syrian troops prepared on Thursday to launch an all-out offensive on rebel-held districts across Aleppo, a security source said, as pro-regime media warned of a looming "mother of all battles."

Fighting raged in parts of Syria's most populous city through the night and into Thursday, but the security source told AFP the main offensive was still to come.

"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," he said.

Rebel fighters had brought in their own reinforcements, with the source estimating that between 1,500 and 2,000 opposition fighters had arrived from outside Syria's largest city to reinforce some 2,000 already fighting in Aleppo.

"They are mainly present in the southern and eastern suburbs of the city, mainly Salaheddin and nearby districts," he said.

The airport is currently cut off from the city, as four of the five roads leading to it are under rebel control, he added.

Rebels too said a regime assault appeared imminent.

"The army's reinforcements have arrived in Aleppo," Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army (FSA) in Aleppo, told AFP via Skype.

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

Okaidi added that some 100 army tanks — as well as a large number of military vehicles — had arrived in Aleppo.

Fighting has flared since rebels launched on July 20 an all-out assault to overrun Syria's commercial hub — a move analysts say is aimed at establishing a bastion inside the country close to the rebel military headquarters in neighbouring Turkey.

At least least 19 civilians, three rebels and an unknown number of soldiers died in fighting in Aleppo on Wednesday, according to the Syrian Observatory for Human Rights.

An AFP correspondent witnessed FSA rebels overrunning a police station Aleppo's Shaar district on Wednesday.

He said at least two policemen were killed, an unknown number wounded and several captured in the clashes that left the station's walls riddled with bullets.

The building was later set alight.

Syrian newspaper Al-Watan, which is close to the regime, led Thursday with the headline "Aleppo, the mother of all battles," adding that "the army continues to chase terrorists in the outskirts of Damascus and the province."

Citing an Arab diplomatic source, the newspaper added: "Aleppo will be the last battle waged by the Syrian army to crush the terrorists and after that Syria will emerge from the crisis."

-- Street battles --

In southern Damascus, street battles were being fought on Thursday in the Yarmouk Palestinian refugee camp, the Observatory said.

"There are clashes on Street 30 in the Yarmouk camp between Syrian regime forces and fighters from rebel units. Explosions can be heard," it said.

A resident of the camp reached by phone confirmed the fighting.

"It started at 7:00 am. The night was quiet. They are using RPGs and heavy machineguns," he told AFP.

Elsewhere in the city, an activist in the southern neighbourhood of Tadamun, who gave his name as Abu Qais al-Shami, said several districts in the southern part of the city were under assault on Thursday by regime forces.

After a week of heavy clashes, activists say regime forces have largely regained control of Damascus, with only a few pockets of resistance remaining.

On the political front, Syria's regime confirmed on Thursday the defection of three diplomats, but downplayed their importance and indirectly accused Qatar of encouraging "national division."

The foreign ministry confirmed the defections of Lamia Hariri, charge d'affaires in Cyprus, her husband Abdel Latif al-Dabbagh, ambassador to the United Arab Emirates and Mohammed Tahsin al-Fakir, security attache in Oman.

"These ministry employees chose to abandon their diplomatic posts and go to a certain Arab capital, which is funding and encouraging these type of staff defections," the ministry said, referring to Qatar, where the diplomats have reportedly fled.

White House spokesman Jay Carney told reporters aboard Air Force One that the moves showed that "senior officials around the Assad inner circle are fleeing the government because of the heinous actions taken by Assad against his own people, and the recognition that Assad's days are numbered."

UN peacekeeping chief Herve Ladsous, meanwhile, said he had told Syrian officials that without a significant reduction in violence, the remaining 150 UN observers would leave on the expiry of the "final" 30-day extension of the mission's mandate, agreed by the Security Council on July 20.

In an updated toll on Thursday, the Observatory said 143 people were killed throughout Syria on Wednesday, including 75 civilians, 41 soldiers and 27 rebel fighters.

According to the Observatory's tally, more than 19,000 people, mostly civilians, have died since Assad sent in his troops to crush a a popular uprising that erupted in March last year.