The United Nations suspended all humanitarian convoys in Syria on Tuesday following a deadly air strike on aid trucks, as fighting intensified after the regime declared an end to a week-long truce.

Both Syria and Russia denied they were behind the raid on the convoy near northern city Aleppo, which the Red Cross said killed "around 20 civilians" including an employee of the Syrian Red Crescent.

Air raids and shelling meanwhile pounded key battlefronts across the country -- dimming hopes that the fraught ceasefire brokered by Moscow and Washington could be revived.

Key players including the United States and Russia were meeting 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.

Kerry and Russian Foreign Minister Sergei Lavrov opened a meeting of the 23-nation International Syria Support Group (ISSG) in New York, where world leaders have gathered for the UN General Assembly.

UN chief Ban Ki-moon opened the assembly debate with a call to end the fighting in Syria.

"I appeal to all those with influence to end the fighting and get talks started," Ban said.

Monday's strike on the aid convoy provoked outrage from UN officials, with aid chief Stephen O'Brien warning that if deliberate "it would amount to a war crime".

The International Federation of Red Cross and Red Crescent Societies said Monday night's raid destroyed at least 18 of 31 vehicles, as well as a Red Crescent warehouse in Orum al-Kubra in Syria's Aleppo province.

"Much of the aid was destroyed," the IFRC said in a statement, stressing that "the attack deprives thousands of civilians of much-needed food and medical assistance."

Omar Barakat, who headed the local Red Crescent branch, was wounded in the strike and later died, IFRC spokesman Benoit Carpentier told reporters in Geneva.

- 'Dark day' for aid workers -

UN humanitarian agency OCHA said that the movements of all aid convoys in Syria had been suspended as an "immediate security measure" after the raid.

The attack marked a "very, very dark day for humanitarians in Syria and indeed across the world," OCHA spokesman Jens Laerke said, adding that it was "paramount that we are able to establish the facts through an independent investigation."

A Syrian military source denied any regime involvement, telling state media: "There is no truth to media reports that the Syrian army targeted a convoy of humanitarian aid in Aleppo province."

The Russian defence ministry also said that both its forces and the Syrian air force "did not conduct any strikes against the UN aid convoy."

The strike came just a few hours after the Syrian army announced the end of the truce on Monday night, accusing rebels of failing to "commit to a single element" of the US-Russia deal.

Heavy fighting almost immediately resumed, with activists and AFP correspondents on the ground in Syria reporting bombardments overnight and on Tuesday in several areas.

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.

- 'Ready for barrel bombs' -

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 monitoring group 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.

In the northwestern province of Idlib Tuesday, activist Nayef Mustafa said planes circled over the opposition-held town of Salqin.

"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.

Kerry and Lavrov had negotiated the truce 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.

Aid to desperate civilians was a key element of the deal, but deliveries were minimal during the truce and cross-border assistance never entered Syrian territory.

The ceasefire was already under massive strain after 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.