It is just a matter of time before the government of Syrian President Bashar al-Assad falls because its use of massive force is mobilizing insurgents, General Robert Mood, the former head of the UN monitoring mission in Syria, said.

‘‘In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall,’’ the Norwegian general, who left Damascus on July 19, said.

‘‘Every time there are 15 people killed in a village, 500 additional sympathizers are mobilized, roughly 100 of whom are fighters,’’ Mood said.

However, the authoritarian Syrian leader is probably secure in the short term because he has the military capability to hold off the rebels and his eventual fall could be months or even years away, Mood said.

‘‘In the short term it may very well be possible for him to (hold on), because the military capabilities of the Syrian army are much much stronger than those of the opposition,’’ Mood said.

‘‘The minute you see larger military formations leaving the ranks of the government to join the opposition, then that is when it starts accelerating ... This could last for months or even years,’’ he said.

Mood left Syria after his 90-day mission expired — the mandate was renewed on July 20 for 30 days — and has been replaced by Lieutenant General Babacar Gaye, who has already taken up his post in Damascus.

Mood, who commanded a 400-strong mission, was unable to stop the escalating violence and said organised groups with artillery, mortars and mechanized formations were responsible for some of the violence in Syrian villages.

MILITARY POUNDS REBEL TERRITORY

Meanwhile, President Bashar al-Assad’s artillery pounded rebel-held areas around Aleppo, preparing the ground for an onslaught on Syria’s biggest city where the United States has said it fears a ‘‘massacre’’ may be imminent.

Opposition sources said the shelling, which follows intensive ground and air bombardment of the city itself, was an attempt to stop fighters from resupplying rebel units inside Aleppo.

‘‘They are shelling at random to instil a state of terror,’’ said Anwar Abu Ahed, a rebel commander outside the city.

The battle for Aleppo, a major power centre that is home to 2.5 million people, is being seen as a potentially game changing turning point in the 16-month uprising against Assad that could give one side an edge in a conflict where both the rebels and the government have struggled to gain the upper hand.

A rebel commander said insurgents had attacked a convoy of Syrian army tanks heading towards the city, as the government continued to redeploy forces from other parts of the country to bolster its forces there.

The fate of Syria itself — an ethnically fragmented nation of 22 million people — is likely to determine the future of the wider region for years to come amid fears that its own sectarian tensions could spill across its porous borders.

The US State Department said credible reports of tank columns moving on Aleppo, along with air strikes by helicopters and fixed-wing aircraft, represented a serious escalation of Assad’s efforts to crush his opponents.

‘‘This is the concern: that we will see a massacre in Aleppo, and that’s what the regime appears to be lining up for,’’ Victoria Nuland, a spokeswoman for the State Department, said.

As the remaining residents of Aleppo braced themselves for more bloodshed, General Robert Mood, the outgoing head of the U.N. monitoring mission, told Reuters he thought Assad’s days in power were numbered.

‘‘In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall,’’ the Norwegian general, who left Damascus on July 19, said.

Navay Pillay, the United Nations human rights chief, said a pattern had emerged as Assad’s forces resorted to shelling, tank fire and door-to-door searches.

‘‘All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city,’’ Pillay said in statement.

MORTAR ROUNDS

Government troops stationed on the outskirts of the city unleashed barrages of heavy-calibre mortar rounds on its western neighbourhoods, while Russian-built MI-25 helicopter gunships struck in the east, opposition activists in the city said.

The heavy fighting follows an audacious bomb attack that killed four of Assad’s closest lieutenants in Damascus on July 18, a development that led some analysts to speculate that the government’s grip was slipping.

In the first reported casualty on Friday, a man of about 60 wearing a traditional white prayer outfit was killed near a park in Aleppo. His body was placed in a mosque pending identification.

On Thursday, thirty-four people were killed in and around Aleppo, according to opposition activists.

‘‘The rebels have so far been nimble, and civilians have mostly been the victims of the bombardment,’’ said activist Abu Mohammad al-Halabi, speaking by phone from the city.

Majed al-Nour, another activist, said rebels had attacked a security outpost in the neighbourhood of Bustan al-Joz, which is close to Aleppo’s city centre, on Thursday.

‘‘The rebels are present in the east and west of the city, and have a foothold in areas of the centre. The regime forces control the entrances of Aleppo and the main thoroughfares and commercial streets and are bombarding the residential districts that fell into rebel hands,’’ he said.

Nour said tens of thousands of people had fled Aleppo to nearby northern rural regions close to Turkey.

- Reuters