President Bashar al-Assad’s forces renewed a ground and aerial bombardment of Aleppo on Friday, trying to crush rebels in Syria’s commercial capital in what the United States said it feared could become a massacre.

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

The State Department said credible reports of tank columns moving on Aleppo, along with airstrikes by airplanes and helicopters, 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 Aleppo unleashed barrages of heavy mortar rounds on the western neighborhoods of Saladin, al-Sukkari and al-Fardos, while Russian-made MI-25 helicopter gunships struck al-Sakhour in the east with rockets, several opposition activists in the city said.

In the first reported casualty Friday, a man of about 60 was killed near a park in Saladin. Thirty-four people were killed in Aleppo and its environs Thursday, according to opposition activists keeping a tally of casualties in the northern city.

“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 Thursday attacked a security outpost in the neighborhood of Bustan al-Joz, which is close to the Aleppo city center.

“The rebels are present in the east and west of the city and have a foothold in areas of the center. 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tens of thousands of people had fled Aleppo to nearby rural regions close to Turkey, from which the Syrian army has withdrawn in recent weeks to focus on urban areas.

One of the most senior figures to defect from Assad’s inner circle, Brig. Gen. Manaf Tlas, offered himself Thursday as someone who could help unify the fragmented opposition inside and outside Syria on a plan for a transfer of power.

Tlas, speaking in a newspaper interview in the Saudi city of Jeddah, also said he was looking for support from Saudi Arabia and other powers. “I am discussing with . . . people outside Syria to reach a consensus with those inside,” Tlas said in Thursday’s edition of the Saudi newspaper Asharq al-Awsat.

Tlas went on to Turkey and met with Foreign Minister Ahmet Davutoglu. Turkey, a former ally of Assad and now one of his fiercest critics, has a heavy strategic stake in shaping any post-Assad leadership. Tlas appeared briefly with Davutoglu at an official guest house, but made no statement.

— Reuters