Missiles rained down on rebel-held areas of Syria's Aleppo on Friday, causing widespread destruction that overwhelmed rescue teams, as the army prepared a ground offensive to retake the city.
Nearly 30 civilians including several children were killed and dozens wounded in the raids by Russian warplanes and regime aircraft, the Syrian Observatory for Human Rights, a Britain-based monitoring group, said.
The intensity of the bombardment, which included artillery barrages and barrel bombings by helicopters, brought new misery to the estimated 250,000 civilians besieged by the army.
The escalation came after US Secretary of State John Kerry failed to reach an agreement with Russian counterpart Sergei Lavrov on terms to salvage a failed ceasefire.
Asked by reporters at the United Nations whether the truce could be reinstated, Lavrov simply said: "You should ask the Americans."
A US official told AFP that no Kerry-Lavrov meeting had yet been scheduled for Friday, "but that could change".
An AFP journalist in rebel-held east Aleppo reported relentless air raids and artillery fire overnight and Friday morning.
Entire apartment blocks were flattened, overwhelming rescue teams from the White Helmets civil defence organisation.
In the Al-Kalasseh district, three buildings were levelled by a single strike, and rescue workers were trying frantically to reach survivors using a single bulldozer and their bare hands.
The White Helmets' headquarters in the Ansari district was badly damaged along with an ambulance and a fire engine. A second centre operated by the group was also hit.
Also in Aleppo province, the Observatory reported 12 deaths in a Russian raid on the rebel-held town of Beshkatine and 11 killed in raids by unidentified aircraft on Islamic State group stronghold Al-Bal.
The bombardment came a day after the Syrian army announced an offensive to recapture east Aleppo, which has been held by the rebels since mid-2012 but has been surrounded by government forces since July.
The army urged civilians to distance themselves from "the positions of terrorist groups" and pledged that fleeing residents would not be detained.
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 AFP.
"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."
Another military source in Damascus said "the goal of the operation will be to expand the area under the army's control".
He said reinforcements had already been brought to Aleppo.
The talks between Kerry and Lavrov in New York on Thursday broke up after Russia refused US demands that it promise to immediately ground the Syrian regime's air force.
"We cannot continue on the same path any longer," Kerry said, adding that "credibility" had to be restored to the process.
"The only way to achieve that is if the ones that have the air power in that part of the conflict simply stop using it."
The conflict in Syria has cost more than 300,000 lives and displaced over half the country's population since it erupted in March 2011.
In a bid to relaunch peace talks, Kerry and Lavrov announced a ceasefire on September 9, with Moscow responsible for forcing government troops to stand down and allow in UN aid convoys.
Washington was supposed to pressure rebel forces to respect the truce and distance themselves from jihadists, but the ceasefire fell apart acrimoniously and the Syrian army declared it over on Monday.
UN peace envoy Staffan de Mistura said Thursday's failed talks were "long, painful and disappointing" and warned of escalating violence.
In Geneva, the UN said Friday it was considering a different route to send desperately needed aid to east Aleppo to circumvent the blocked main supply route.
In Damascus, an analyst close to the regime said it was no coincidence that the Aleppo assault began as the New York talks broke down.
"In Aleppo, negotiations are being conducted by fire," he said.
"The Americans must understand that so long as they don't implement their commitments, particularly for the rebels to distance themselves from... (jihadists), the Russians and the Syrian army will advance."