A Syrian family walks amid rubble after an airstrike on Sept. 23 in a rebel-held area of Aleppo. (Thaer Mohammed/AFP/Getty Images)

Airstrikes pounded rebel-held areas in the embattled Syrian city of Aleppo on Friday, hitting centers for a volunteer civil defense group in a sharp escalation by government forces after the collapse of cease-fire plans that raised fleeting hopes of peace.

The intensifying offensive on Aleppo — a critical foothold for rebel groups — came amid signals of an all-out push by President Bashar al-Assad to reclaim full control over the northern city, which remains virtually cut off from medical and food supplies.

Activists claim the latest air attacks have tried to further cripple the limited resources in the rebel zones. Among the targets, they say, have been the operational hubs for the civil defense group known as the White Helmets, whose teams rush to bombing sites to aid survivors.

At least three centers had been hit by airstrikes, and fire trucks and ambulances have been damaged, Ibrahim Alhaj, a member of the group, told the Associated Press.

“I have not seen in my life such bombardment. It is very, very intense,” he said.

The latest cease-fire in Syria lasted six days before the warplanes returned to Aleppo. Eyewitness accounts claim white phosphorus munitions were dropped on the city on Sept. 20. The incendiary weapon burns at extreme temperatures. (TWP)

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the Syrian troops had pushed back rebel lines in a southern district of Aleppo. More than 70 airstrikes have hammered parts of Aleppo since Wednesday, the group said.

For Assad, the city represents an important prize that would expand government control in the north and deprive opposition groups of one of its last main strongholds.

In a possible hint of the looming showdown in Aleppo, Syrian state media said a “ground offensive” would eventually be launched, citing a Syrian military official whose name was not given.

On Thursday, the White Helmets rescue service said three of its four centers had been hit by bombs, knocking two of out commission.

The announcement of the Syrian offensive suggested that Syria’s government has no intention of complying with any further cease-fire requests from the international community, despite appeals by Secretary of State John F. Kerry the day before to revive the failed attempt to stop the fighting.

The cease-fire plan, brokered by the United States and Russia, sought to open routes for humanitarian aid into besieged areas such as Aleppo. It brought a lull in fighting for several days last week, but aid convoys never reached the needy amid hold-up for clearance from Assad’s government.

Amateur video purports to show the aftermath of Syrian government airstrikes in Aleppo’s al-Ansari neighborhood in some of the heaviest strikes yet on rebel-held areas. (Reuters)

In an interview with the Associated Press in Damascus, a defiant Assad said he takes no notice of what U.S. government officials say.

“American officials — they say something in the morning and they do the opposite in the evening,” he said. “You cannot take them at their word, to be frank. We don’t listen to their statements, we don’t care about it, we don’t believe it.”

Scores of people have been killed in the airstrikes since the cease-fire collapsed on Monday, including at least 30 in Aleppo, activists said. The reports could not be independently verified, but images on social media chronicled the extent of the latest air attacks.

“This means welcome to hell,” said Abdulkafi Al-Hamdo, a teacher who lives in rebel-held Aleppo. “We expect extermination.”

The government, meanwhile, claimed victory over another small corner of the country, in the central city of Homs. Some 300 rebels and their families piled onto buses in the neighborhood of Al-Waer on Thursday after accepting the terms of government surrender deal to leave their homes in return for safe passage to rebel-held territory further north.

The official Syrian news agency SANA said Russian troops — which are backing Assad — helped supervise the evacuation, which has been condemned by the United Nations and the Syrian opposition as a form of forced displacement.

The capitulation of the rebels in Waer means that the city of Homs is now entirely under government control for the first time in nearly five years. The deal was similar to others that have been implemented in neighborhoods elsewhere that had joined the original revolt against Assad only to find themselves surrounded by government troops and cut off from food and medical supplies. The Syrian opposition and the United Nations have condemned the surrender deals, proclaimed as forced displacement, but they have proved an effective way for the government to slowly reassert its authority in areas that had slipped beyond its control during the rebellion.

Murphy reported from Washington. Liz Sly in Beirut contributed to this report.