Air raids and shelling pounded key battlefronts in Syria on Tuesday, as outrage mounted over a strike on an aid convoy hours after Syria's military declared an end to a week-long truce.
The United Nations said it was suspending all humanitarian aid convoys in Syria after the deadly raid late Monday, which killed several civilians including a senior employee of the Syrian Red Crescent.
The strike and renewed violence across the country dimmed hopes that the fraught ceasefire negotiated by Moscow and Washington could be revived.
Key players including the United States and Russia were to meet in New York Tuesday in an effort to salvage the peace process, which US Secretary of State John Kerry had warned could be the "last chance" to end Syria's civil war.
But on the ground in Syria, activists and AFP correspondents reported intensifying fighting.
In the battleground city of Aleppo, air raids and artillery fire hit rebel-held districts until approximately 2:00 am (2300 GMT Monday), an AFP correspondent said.
Residents spent the night huddled in their apartments sharing news about the collapsing truce via text messages and heard loud intermittent booms on Tuesday morning.
At least 39 civilians were killed in overnight bombardment of Aleppo and the surrounding province, the Syrian Observatory for Human Rights said, and fresh clashes erupted on the city's southern edges.
In the week after the truce was declared on September 12, only 27 civilians were killed as fighting dropped significantly across the country.
Monday night's raid on the convoy destroyed at least 18 of 31 vehicles delivering aid under a joint UN, Red Cross, and Red Crescent operation for the town of Orum al-Kubra in Aleppo province.
The Observatory, a Britain-based monitoring group, said 12 Red Crescent volunteers and drivers had died in the strike.
Jens Laerke, spokesman for the UN humanitarian agency, told reporters in Geneva that as an "immediate security measure, other convoy movements have been suspended" following the raid.
This marks a "very, very dark day for humanitarians in Syria and indeed across the world," he said, stressing that it was "paramount that we are able to establish the facts through an independent investigation."
Red Cross spokesman Benoit Carpentier told reporters that a senior official of the Syrian Red Crescent was among those killed.
Infuriated UN officials had earlier warned the strike could amount to a war crime.
"If this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," aid chief Stephen O'Brien said.
An AFP correspondent at the scene of the strike early Tuesday saw damaged boxes of medical supplies and bags of aid spilling out of charred green trucks.
Aid to desperate civilians was a key element of the US-Russia deal, but deliveries were minimal during the truce and cross-border assistance for besieged civilians in eastern parts of Aleppo city never entered Syrian territory.
Syria's military unilaterally announced the end of the truce on Monday night, accusing rebels of failing to "commit to a single element" of the deal.
In the northwestern province of Idlib Tuesday, activist Nayef Mustafa said planes circled over the town of Salqin, held by Syria's former Al-Qaeda affiliate in alliance with Islamist rebels.
"It's calm now, but there was machinegun fire by military aircraft overnight," Mustafa told AFP.
"The ceasefire has collapsed and people are getting ready to be hit by barrel bombs."
At least four air strikes hit the central rebel-held town of Talbisseh on Tuesday morning after artillery fire throughout the night, activist Hassaan Abu Nuh said.
The truce deal's primary sponsors, Washington and Moscow, and other key players in the International Syria Support Group were to meet in New York on Tuesday to assess the situation.
Kerry is expected to try to speak to Russian Foreign Minister Sergei Lavrov in New York, where leaders are gathered for the UN General Assembly, before Tuesday's meeting.
The two top diplomats had negotiated the deal earlier this month, hoping to put an end to more than five years of conflict in which more than 300,000 people have been killed.
The truce was already under massive strain when a US-led coalition strike on Saturday hit a Syrian army post near the eastern city of Deir Ezzor, where government forces are battling the Islamic State jihadist group.
In Moscow, Kremlin spokesman Dmitry Peskov said the truce could only resume if attacks ceased on forces loyal to President Bashar al-Assad.
"The conditions are very simple. The shooting needs to stop and the terrorists need to stop attacking Syrian troops," Peskov said.
"And of course it wouldn't hurt if our American colleagues didn't accidentally bomb the Syrians."
He said Russia was "verifying information" about the strike on the aid convoy and warned against drawing "unsubstantiated conclusions".