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.
Forty-five civilians including several children were killed and dozens wounded in the raids on eastern Aleppo by Russian warplanes and regime aircraft, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
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 Thursday on terms to salvage a failed ceasefire.
The two met again on Friday at the United Nations and made what Kerry said was "a little bit of progress" on resolving their differences on Syria.
"We're evaluating some mutual ideas in a constructive way, period," Kerry told reporters.
Asked at the UN earlier whether the truce could be reinstated, Lavrov simply said: "You should ask the Americans."
He later told the UN General Assembly that US-Russian agreements aimed at ending the Syria conflict must be salvaged, saying there was "no alternative" to the process.
"Now it is essential to prevent a disruption of these agreements," Lavrov said.
Thursday's Kerry-Lavrov talks in New York broke up after Russia refused US demands that it promise to immediately ground the Syrian regime's air force.
Also in New York, French Foreign Minister Jean-Marc Ayrault accused Syrian President Bashar al-Assad of "playing the card of a partition" of his country with the Aleppo offensive.
The Syrian opposition coalition, meanwhile, condemned what it termed the regime's Russian-backed "criminal campaign ... targeting the besieged residential districts of Aleppo".
An AFP journalist in rebel-held east Aleppo reported relentless air raids and artillery fire overnight and Friday morning.
Medical charity Doctors Without Borders (MSF), which supports hospitals in Aleppo, said the city's residents "already suffocating under the effects of the siege, have yet again come under horrific attack".
"In many areas, the wounded and sick have nowhere to go at all -– they are simply left to die," said Carlos Francisco, the MSF head of mission in Syria.
An AFP correspondent inside the city said the barrage had flattened entire apartment blocks, overwhelming rescue teams from the White Helmets civil defense organization.
In the Al-Kalasseh district, three buildings were levelled by a single strike, and rescue workers tried frantically to reach survivors using a single bulldozer and their bare hands.
The White Helmets' headquarters in the Ansari district was badly damaged and a second centre operated by the group was also hit.
Rescue workers told AFP their stock of diesel was down to 2,000 litres (530 gallons), forcing them to ration fuel and make choices on when to intervene.
Also in Aleppo province, the Observatory reported 15 deaths including 11 children 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."
The conflict in Syria has cost more than 300,000 lives and displaced over half the country's population since March 2011.
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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.
But Lavrov said in New York: "We will not be able to improve the humanitarian situation without the rooting out of the terrorist groups."