Syrian army jets have launched more than 150 air strikes on eastern Aleppo in the past 24 hours, killing at least 90 people in the northern city and its countryside, according to residents.
The latest raids on Friday, part of a new Syrian government offensive to recapture rebel-held parts of Aleppo, destroyed emergency service structures, as well as underground shelters used by civilians to hide from bombings.
We feel the earth trembling and shaking under our feet. Aleppo is burning
At least 30 neighbourhoods were targeted, Al Jazeera's Amr al-Halabi reporting from the city said, adding that the relentless bombardment was hampering the ability of rescue workers to help civilians caught up in the fighting.
Three centres for a volunteer rescue group known as the White Helmets were also hit in the raids.
"We have four centres in eastern Aleppo. The aircraft targeted three centres. Two of them are now out of service," Abdul Rahman al-Hassani, of the White Helmets, told Al Jazeera.
He added that five vehicles belonging to the White Helmets were destroyed, including an ambulance.
"Our centres were the direct target [of the strikes]," Hassani said.
It was the second consecutive day of such fierce attacks, with one resident saying that the bombing continued all night and into the morning.
"We feel the earth trembling and shaking under our feet. Aleppo is burning," Bahaa al-Halabi, an activist from a besieged rebel-held district, told DPA news agency.
"People are not safe any more, even in shelters," resident Yassin Abu Raed said. Other locals said the latest raids were destroying underground shelters people had built.
The Syrian military, which is backed by the Russian air force, said late on Thursday that it was starting a new operation against the rebel-held east, which is home to at least 250,000 people and has been battered by intense bombing for months.
In a statement on its official website, the Syrian defence ministry called on Aleppo residents to move to government-held areas, adding that there would be "no detention, or inquiry to any citizen" who reached the checkpoints that divide the city.
A high-ranking military source confirmed that the bombardment was preparation for a ground assault.
"We have begun reconnaissance, aerial and artillery bombardment," he told the AFP news agency.
"This could go on for hours or days before the ground operation starts. The timing of the ground operation will depend on the results of the strikes and the situation on the ground."
Aleppo was once Syria's commercial and industrial hub but has been ravaged by fighting and roughly divided between government control in the west and rebel control in the east since mid-2012.
Rebel districts have been under siege by the army for most of the past two months after troops overran the last supply lines.
READ MORE: Why Aleppo matters
The announcement of the new Syrian army offensive on Thursday came just as international powers failed to revive a collapsed ceasefire during diplomatic talks in New York.
Speaking after a meeting of the International Syria Support Group (ISSG) on Thursday, US Secretary of State John Kerry said that he was determined to restore the ceasefire, calling on the Syrian President Bashar al-Assad and his ally Russia to "do their part" to ease the suffering of the Syrian people.
A truce deal hammered out by Russia and the United States briefly halted the violence earlier this month, but it collapsed after just a week without any of the promised deliveries of desperately-needed relief supplies.
On Monday, a UN aid convoy was attacked, in what the US says was a Russian air strike, though Moscow rejects the charges. At least 20 people were killed and 18 trucks destroyed.
"What is happening is Aleppo is under attack and everyone is going back to the conflict," UN envoy Staffan de Mistura said.
The Syrian civil war started as a largely unarmed uprising against Assad in March 2011, but quickly escalated into a full-blown armed conflict.
Five years on, more than 400,000 Syrians are estimated to have been killed, and almost 11 million Syrians - half the country's prewar population - have been displaced from their homes.
Source: Al Jazeera News And Agencies