Shells and bombs rained down on rebel-held eastern Aleppo on Monday soon after Syria\’s army declared an end to a week-long ceasefire agreed between Russia and the United States.
Damascus and its ally Moscow blamed rebels for the collapse of the truce, but Washington said the terms had not been met for a key aspect of the deal — US-Russia cooperation against jihadists.
The US, Russia and other key players are still set to gather Tuesday in New York for talks aimed at ending the five-year conflict that has killed more than 300,000 people and displaced millions.
Stakes had been high when the ceasefire began on September 12, with US Secretary of State John Kerry warning at the time that it could be the "last chance" to save the country.
But it unravelled about an hour before it had been due to expire on Monday night.
An AFP correspondent in Aleppo reported that the northern city was being pummelled.
Sirens wailed as ambulances zipped through the eastern rebel-held half of the divided city, the correspondent said, describing the bombardment as "non-stop".
The Russian military said jihadists had launched a major attack on a government position on Aleppo\’s southwestern outskirts, forcing regime troops to respond.
"The attack by the terrorists was proceeded by a massive artillery bombardment… from tanks and rocket systems," it said.
Syria\’s military announced the end to the truce, accusing rebels of more than 300 violations and failing to "commit to a single element" of the US-Russia deal.
Kerry reacted testily to the declaration, but implied there was time to save the deal.
"It would be good if they didn\’t talk first to the press but if they talked to the people who are actually negotiating this," said the top US diplomat.
"And I think it\’s, as I said yesterday, time to end the grandstanding and time to do the real work of delivering on the humanitarian goods.
"So we just began today to see real movement of humanitarian goods, and let\’s see where we are. We\’re happy to have a conversation with them," he said, of the Russian side.
Moscow appeared to bury hopes that the truce would last, however.
"Considering that the conditions of the ceasefire are not being respected by the rebels, we consider it pointless for the Syrian government forces to respect it unilaterally," said Lieutenant General Sergei Rudskoy.
Under the agreed ceasefire, fighting was to halt across Syria and humanitarian aid would reach desperate civilians — particularly in devastated eastern Aleppo.
And if it held, the US was to have set up a joint military cell with Russia to target jihadists.
But after four days of relative calm, violence escalated.
The ceasefire came under massive strain on Saturday when a US-led coalition strike hit a Syrian army post near the eastern city of Deir Ezzor, where government forces are battling the Islamic State jihadist group.
Syrian President Bashar al-Assad on Monday blasted the air strikes which he said showed world powers supported "terrorist organisations" like IS.
"The latest example of this is the flagrant American aggression on one of the Syrian army\’s positions in Deir Ezzor," said Assad.
His adviser Buthaina Shaaban went further, telling AFP that Damascus believed the raid which killed at least 62 Syrian soldiers had been "intentional".
The bloodiest day for civilians was Sunday, when a barrel bomb attack killed 10 in a southern rebel-held town and one woman died in the first raids on Aleppo since the truce started.
Violence increased on Monday, with fierce clashes reported east of Damascus and one child killed in regime shelling on the edges of Aleppo.
Since September 12, 27 civilians, including nine children, have been killed in areas where the truce had been set to take hold, said the Syrian Observatory for Human Rights.
Late on Monday night, air strikes hit around 20 humanitarian aid trucks outside a Syrian Arab Red Crescent centre in northern Aleppo province, a monitor said.
It was not immediately clear if there were any casualties in the attack on the Orum al-Kudra area, said the Observatory.
Some aid got through, with the UN saying convoys had reached tens of thousands in rebel-held Talbisseh and another 78,000 people in and around Greater Orum.
But convoys to rebel-held districts of Aleppo, besieged by government troops, were still stuck on the border with Turkey.
The United States, Russia and other key players in the Syria peace process are to meet Tuesday in New York, said the US State Department.
Foreign ministers from the International Syria Support Group comprised of some 20 countries including Saudi Arabia and Turkey would assess the situation, said deputy spokesman Mark Toner.
The meeting comes ahead of a UN Security Council session on Syria to be held Wednesday.