Fighting raged in Syria's second city on Thursday as troops and rebels prepared for a head-on confrontation and pro-regime media warned of a looming "mother of all battles."

A security source said the army was preparing for an all-out assault as clashes also shook parts of Damascus and other areas, with at least 114 people reported killed on Thursday -- 61 civilians, 32 regime troops and 21 rebels.

"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," the security source said of Aleppo.

Rebels also brought in reinforcements, with the source estimating that between 1,500 and 2,000 opposition fighters had arrived from outside Syria's most populous city to reinforce some 2,000 already fighting inside 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 also said a regime assault appeared imminent.

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

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

Okaidi added that some 100 tanks and a large number of military vehicles had arrived in Aleppo, the country's commercial hub.

The Britain-based Syrian Observatory for Human Rights reported that regime forces pounded the Salaheddin neighbourhood in the south and Jazamati in the east.

On July 20 the rebels launched an all-out assault to overrun Aleppo a move analysts say is aimed at establishing a bastion close to the rebel military headquarters in neighbouring Turkey.

The newspaper Al-Watan, which is close to the regime, led on 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, it 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."

As UN heritage agency UNESCO called for forces fighting in Syria to avoid damaging cultural sites, in particular in world heritage site Aleppo, UN chief peacekeeper Herve Ladsous said there was "no plan B."

"There is one political process for the time being, that is the six-point plan of the Joint Special Envoy Kofi Annan," he told reporters in Damascus.

"And as you know, and has been said time and again, there is no plan B. There is no alternative to that."

Annan's plan called for an inclusive political process, a ceasefire, humanitarian aid, release of detainees, freedom of movement for journalists and peaceful demonstrations to be allowed.

"Syrians killing Syrians is something that should not continue," Ladsous added.

Street battles

Intermittent clashes were also reported in the Damascus area, with seven people killed there and 16 others, including five children, killed in shelling on Yalda village just south of the capital, the Observatory said.

It reported clashes in the Yarmuk Palestinian refugee camp, where a resident reached by phone confirmed the fighting.

Troops also pounded several other districts in southern Damascus, and heavy clashes were under way in Al-Hajar Al-Aswad neighbourhood as regime helicopters strafed the area, activists and residents said.

"Last night was quiet but people woke up to the sound of explosions and shelling from seven o'clock in the morning," an activist calling himself Abu Qais al-Shami said.

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

Meanwhile, Israel on Thursday boosted security along its ceasefire line with Syria in the occupied Golan Heights, an Israeli source said on condition of anonymity.

"The Israel army is reinforcing the fence between Israel and Syria, by adding more barbed wire," he said.

Syria's foreign ministry confirmed the defection of three diplomats, but downplayed their importance and indirectly accused Qatar -- where they reportedly fled -- of encouraging "national division."

It named them as 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.

White House spokesman Jay Carney said the defections show "the recognition that Assad's days are numbered."

And French Foreign Minister Laurent Fabius said on Thursday Assad's regime is bound to fall.

"I'm confident that sooner or later, Bashar will fall," he said at a Warsaw news conference, describing the actions of the regime as "abominable behaviour."

A European Commission aid expert said on Thursday the humanitarian situation in Syria has taken a dramatic turn for the worse.

"It's like running behind a train that constantly keeps accelerating," said the official, adding that more funds were needed to assist refugees fleeing to neighbouring Iraq, Lebanon, Jordan and Turkey.

Tens of thousands of people have fled Syria to escape the violence which the Observatory says has killed more than 19,000 people since mid-March 2011.