BEIRUT: United States said it feared a massacre in Aleppo after President Bashar al-Assad's forces launched ground and air bombardments in preparation for a major onslaught against rebels in Syria's commercial capital.

Insurgents targeted army roadblocks and security installations, with both sides avoiding close-quarters warfare in the northern city of 2.5 million people, Syria's biggest urban centre.

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 a rebellion that began 16 months ago.

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

Troops stationed on the outskirts of the city unleashed barrages of heavy-calibre mortar rounds on the western neighbourhoods of Saladin, al-Sukkari and al-Fardos, while Russian-built MI-25 helicopter gunships struck al-Sakhour in the east with rockets, opposition activists in the city said.

The heavy fighting around Aleppo follows an audacious bomb attack that killed four of Assad's closest lieutenants in Damascus on July 18 and 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 Saladin. His body was placed in a mosque pending identification.

Thirty-four people were killed in and around Aleppo on Thursday, 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.

“There is lots of internal displacement, and schools have been turned to makeshift shelters that are packed. One shell hitting a school will result in a catastrophe,” he said.

“The regime is massing troops and tanks at the entrances of Aleppo, but it seems it is for now content with bombarding the city, with the rebels constantly on the move.”

Majed al-Nour, another activist, said rebels attacked a security outpost in the neighbourhood of Bustan al-Joz, which is close to the Aleppo 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 from which the Syrian army has withdrawn in recent weeks to focus on urban areas where relatively lightly armed rebels have hunkered down.

With UN Security Council resolutions for sanctions against Syria vetoed by Russia and China for a third time last week, the United States has said it is stepping up assistance to Syria's fractured opposition, although it remains limited to non-lethal supplies such as communications gear and medical equipment.

Reuters has learned that the White House has crafted a presidential directive, called a “finding,” that would authorize greater covert assistance for the rebels, while still stopping short of arming them.

It is not clear whether President Barack Obama has signed the document, and US officials declined to comment on the finding, which is a highly classified authorization for covert activity.