Russia and the United States clashed at the United Nations over the carnage in Syria on Wednesday, as air strikes pounded Aleppo following the collapse of a ceasefire.

An angry US Secretary of State John Kerry demanded at a UN Security Council meeting that Russia force Syria to ground its air force, which Washington blames for an attack on an aid convoy.

Kerry and his Russian counterpart Sergei Lavrov are to sit down with key players in the conflict on Thursday to try to revive the ceasefire and chart a new course towards ending the five-year war.

"We are at a make or break moment," UN Secretary General Ban Ki-moon told the council, urging world powers to use their influence to help restart political talks so Syrians can "negotiate a way out of the hell in which they are trapped."

Russia and the United States negotiated the latest ceasefire plan, but Syria ended the truce on Monday following an apparently accidental US-led coalition strike on Syrian soldiers.

Shortly after the truce ended, the UN aid convoy was hit, killing 20 humanitarian workers and destroying 18 trucks carrying food for desperate civilians in Aleppo province.

On Wednesday, heavy bombardment pummeled Aleppo city and the wider province, key battlegrounds in Syria's conflict, and a raid hit a medical team late Tuesday.

Addressing the council, Kerry said the bombing of the aid trucks raised "profound doubt" about whether Russia and its Syrian ally were committed to upholding a ceasefire.

"We must move forward to try to immediately ground all aircraft flying in those key areas in order to de-escalate the situation and to give a chance for humanitarian assistance to flow unimpeded," he said.

Moscow denies that Russian or Syrian planes carried out Monday's strike on the aid convoy.

A Russian military spokesman said a coalition drone was in the area when the aid trucks were destroyed, a claim the Pentagon denied.

After halting aid operations in response to the convoy attack, the United Nations said it was ready to resume humanitarian deliveries.

- Diplomacy still in play -

Despite the US-Russian acrimony, diplomatic efforts were set to continue in New York with a new meeting of the International Syria Support Group (ISSG) to be held on Thursday.

The ISSG, which includes all key players in the Syria conflict, met for an hour on Tuesday, but made little headway in agreeing on the next steps to end the war that has killed 300,000 people.

Sounding a cautious note, Kerry told reporters late Wednesday that "it's going to be difficult. We'll see what people are willing to do."

Moscow meanwhile said it is dispatching its flagship aircraft carrier, the Admiral Kuznetsov, to bolster its forces in the eastern Mediterranean off Syria.

In his address to the Security Council, Lavrov declared that there would be "no more unilateral pauses" by Syrian government forces, arguing that opposition fighters on the ground had previously used those ceasefires to regroup.

The foreign minister insisted that all sides must rein in rebel groups on the ground to ensure they comply with the ceasefire and said a list of terror groups not covered by the truce should be reviewed.

Only the Islamic State group and the Al-Qaeda-linked Nusra Front are not covered by the ceasefire, but Russia has long argued that other groups are fighting alongside those jihadists.

"If we can agree on this kind of comprehensive approach, an integrated, multifaceted approach, the chances of a cessation of hostilities surviving and being successful will be better," Lavrov said.

After hearing Lavrov, Kerry said he felt like the Russians were "sort of in a parallel universe" while the Russian foreign minister said it was time to "refrain from emotional reactions."

Syria's Ambassador Bashar Jaafari railed against a "filthy propaganda war" waged by the United States and its allies, insisting his government is fighting "tens of thousands of terrorists" backed by foreign countries.

- Aleppo under fire -

Dozens of raids hit the city's east as regime troops advanced on rebels in Aleppo's southwestern outskirts, according to the Britain-based Syrian Observatory for Human Rights.

At least 12 civilians, including two children, were killed, the highest single-day toll since the truce collapsed this week, the group said.

In northwest Idlib province, 18 Syrians were killed during bombings on the town of Khan Sheikhun, according to the organization.

In the village of Khan Tuman, south of Aleppo city, two nurses and two drivers were killed in an attack on two ambulances, the Union of Syrian Medical Relief Organizations said.

Syrian state media reported that the city's government-held west had come under rebel shelling, which killed two people.

The United States holds Russia responsible for the attack on the aid convoy, with a US official saying two Russian SU-24 ground attack jets were operating in the area where it was struck.

The Russian foreign ministry said the "unsubstantiated, hasty accusations" seemed designed to "distract attention from the strange 'error' of coalition pilots."

Dozens of Syrian troops were killed in Saturday's strike by the US-led coalition fighting Islamic State, which Washington said was a mistake.

Russia said it endangered the deal reached with Washington that provides for a ceasefire, humanitarian aid deliveries and cooperation between Moscow and Washington in battling IS and other extremist groups.

Destroyed aid convoy in Syria: what we know
Paris (AFP) Sept 21, 2016 - Almost 48 hours after a deadly attack on an aid convoy in Syria that killed about 20 civilians, Russia still furiously denies that its jets or Syrian regime planes were responsible.

Here is what we know about Monday's attack that forced the United Nations to suspend its aid deliveries inside the war-torn country:

- What happened? -

Trucks carrying food and medical equipment from the United Nations and other agencies were unloading aid into a warehouse in Orum al-Kubra, a town in Aleppo province.

Jens Laerke, a spokesman for the UN humanitarian agency (OCHA) in Geneva, said: "Every single partner or party to the conflict" was informed in advance about the delivery.

According to the Red Cross, the first report that the trucks and warehouse had been hit came at 8:15 pm (1715 GMT), several hours after the Syrian army had declared an end to a week-long partial ceasefire.

It said "around 20 civilians" had been killed, among them volunteers, as well as Syrian Arab Red Crescent (SARC) staffer Omar Barakat.

Pictures of the convoy published on Twitter by SARC showed several of the trucks draped with blue flags from UNICEF, the UN children's agency.

The raids destroyed at least 18 of the 31 vehicles, the warehouse and much of the aid, the Red Cross said.

The trucks were carrying sanitation and nutritional supplies for 50,000 people, and nine tonnes of medical aid, including antibiotics and surgical materials, according to UNICEF and the World Health Organization.

- Who was responsible? -

The Syrian Observatory for Human Rights, a monitor group, reported Monday night that an air strike had hit the trucks, with a Red Cross spokesman also referring to an air strike.

Although not confirmed, it meant that either Russian or Syrian aircraft were strongly suspected of being responsible as neither opposition groups nor jihadist groups have air power.

Speaking to AFP, an unnamed US official said Washington believed a Russian plane was responsible, saying two Russian SU-24 warplanes had been operating in the area.

"We have a very clear picture of what is going on in the skies over Syria," the official said.

"The reported strike occurred when a Russian plane was directly overhead."

But both Russia and Syria have denied responsibility for the attack, with Moscow furious at the "unsubstantiated, hasty accusations" from the United States.

The Russian military has instead appeared to cast doubt on whether an attack took place, saying footage from activists at the scene showed damage to the vehicles that did not appear to be caused by an air strike or other munitions.

Russia said its drones had tracked the convoy until it successfully delivered its load, hours before the attack, saying a "fire" that destroyed the trucks which "strangely" coincided with a jihadist attack in the area.

But Russian military analyst Pavel Felgenhauer said it was impossible for Moscow to draw any conclusions from amateur video of the incident and only an investigation "on the ground" could determine what destroyed the convoy.

US Secretary of State John Kerry, speaking at the United Nations, said Russia's claims were "contradicted by public information."

Moscow later released drone footage it said showed a vehicle towing a mortar next to the aid convoy, and the Russian military has also claimed a US-led coalition Predator drone was above the area of the convoy attack -- a claim the Pentagon quickly denied.

Russia's military did announce its planes had destroyed 16 rebel vehicles elsewhere around Aleppo as it helped Syrian forces beat back a jihadist attack on Monday evening.

On Tuesday, the Red Cross issued a statement with slightly-altered language which referred to an "attack" on the convoy, and not specifically an air strike.

The main Syrian opposition group said Tuesday it was certain that either Syrian or Russian planes were responsible.

"No one else has aircraft in that area," said leader Riad Hijab in New York.

Hijab said a member of his High Negotiations Committee (HNC) who was escorting the convoy, had provided him with photographs of the attack that he described as "very precise."

International Committee of the Red Cross head Peter Maurer said an investigation was needed into a "flagrant violation of international humanitarian law."

- What is the fallout? -

Laerke said the UN had suspended convoy movements for security reasons, adding that future aid deliveries would depend on a "further assessment of the security situation."

But the Red Cross has not paused deliveries, spokesman Benoit Carpentier told AFP.

Stalled aid deliveries have wreaked havoc across Syria, with the UN, the Red Cross and others repeatedly demanding unhindered access to civilians in desperate need of life-saving supplies.

Even before the strikes, some 40 UN trucks carrying relief supplies destined for rebel-held east Aleppo had remained stuck in a customs zone between the Turkish and Syrian borders since early last week.