Update 10.38am: The UN humanitarian aid agency has temporarily suspended all convoys in Syria following a deadly air strike on aid trucks the previous night.
Spokesman Jens Laerke, of the Office for the Coordination of Humanitarian Affairs (OCHA), said the temporary suspension of the aid deliveries would hold pending a review of the security situation in Syria.
Mr Laerke said yesterday that the UN aid coordinator had received needed authorisations from the Syrian government in recent days to allow for aid convoys to proceed within Syria.
He said it's "a very, very dark day... for humanitarians across the world".
The attack late last night came just hours after the Syrian military declared the week-long US-Russian brokered ceasefire had failed.
The United States said it was prepared to extend the truce deal and Russia - after blaming rebels for the violations - suggested it could still be salvaged.
A United Nations humanitarian aid convoy in Syria has been hit by air strikes with reports of many dead and seriously wounded as the Syrian military declared that a US-Russian brokered ceasefire had failed.
The US initially brushed off Damascus' assertions and said it was prepared to extend the agreement, while Russia - after blaming rebels for the violations - suggested the truce could still be salvaged.
But late last night US State Department spokesman John Kirby called the convoy attack an "egregious violation" of the week-long ceasefire and said America "will reassess the future prospects for co-operation with Russia".
UN humanitarian chief Stephen O'Brien said initial reports indicated many were killed or seriously injured in the convoy attack, including Syrian Arab Red Crescent volunteers.
A Red Crescent warehouse was also hit and a Red Crescent health clinic was reported to be seriously damaged, he said.
Mr O'Brien called the attacks "sickening" and said he was "disgusted and horrified".
He stressed that all parties were notified of the convoy, which was carrying aid for about 78,000 people.
PHOTO: First daylight pictures of the aid trucks hit by Russian/Syrian regime airstrikes last night - @markito0171 pic.twitter.com/Cp0YgokzEJ— Gissur Simonarson 🇮🇸🏴 (@GissiSim) September 20, 2016
UN officials said the world body and Red Crescent convoy was delivering assistance to the town of Uram al-Kubra, west of Aleppo city. Initial estimates indicated that at least 18 of the 31 lorries in the convoy were hit.
The British-based Syrian Observatory for Human Rights said at least 12 were killed in the attack, mostly lorry drivers and Red Crescent workers.
The Syrian Civil Defence, the volunteer first responder group also known as the White Helmets, posted images of a number of vehicles on fire.
A video of the attack showed huge balls of fire in a pitch-black area as ambulances arrived on the scene.
Jan Egeland, humanitarian aid co-ordinator in the office of the UN envoy for Syria, said the convoy was "bombarded".
"It is outrageous that it was hit while offloading at warehouses," he said.
Mr O'Brien said there is no excuse "for waging war on brave and selfless humanitarian workers" and warned that if they were targeted deliberately "it would amount to a war crime".
Elsewhere at least 20 civilians, including a one-year-old girl, were killed in fresh air strikes on rebel-held parts of Aleppo city and surrounding areas, according to the Observatory.
And Russia said government positions in south-western Aleppo came under attack from militant groups, including a massive barrage of rockets.
The week-old ceasefire had brought a brief respite to at least some parts of the war-torn country.
Its future will be at the top of the agenda of a meeting on Tuesday of about 20 countries supporting opposing sides in the conflict, including the US and Russia, that belong to the International Syria Support Group.
In the wake of the Syrian military declaration, US secretary of state John Kerry acknowledged that the first stage of the truce - which called for a week of calm and the delivery of humanitarian aid to several besieged communities - had never really come to fruition.
Earlier Mr Kerry told reporters on the sidelines of the UN General Assembly that the truce was "holding but fragile".
The State Department initially said it was ready to work with Russia to strengthen terms of the agreement and expand deliveries of humanitarian aid and Mr Kirby called on Russia, which is responsible for ensuring Syria's compliance, to clarify the Syrian position.
A Russian Foreign Ministry statement late last night appeared to signal that the deal could still be salvaged, saying that the failure by the rebels in Syria to respect the ceasefire threatened to thwart the agreement.
The ceasefire came into effect on September 12.
Under terms of the agreement, the successful completion of seven days of calm and humanitarian aid deliveries would be followed by an ambitious second-stage plan to set up a joint US-Russian co-ordination centre to plan military strikes against the Islamic State group and a powerful al Qaida-linked militant faction.
But from the start, the truce has been beset by difficulties and mutual accusations of violations.
Aid deliveries to the besieged eastern districts of Aleppo have not reached their destination. The UN accused the government of obstructing the delivery while Russian officials said rebels opened fire at the delivery roads.
Rebel forces and activists say government planes have bombed areas that are under the truce agreement, including rebel-held parts of Aleppo.
At least 22 civilians were killed in government bombings over the last week, according to the Syrian Observatory, and four civilians were killed in government-held areas. There were no independent reports of civilian deaths on the government side since the ceasefire came into effect.
By Monday, both the Syrian government and prominent opposition activists were speaking of the truce as if it had already failed.
George Sabra, of the opposition High Negotiations Committee, said the ceasefire had been broken repeatedly and did not succeed in its main objective or opening roads for aid.
"Hundreds of thousands of people in Aleppo are waiting for this truce to allow aid to enter the city," he said. "I believe that the truce is clinically dead."
The Syrian military statement blamed rebel groups. Damascus refers to all armed opposition groups as terrorists.
"This step (ceasefire) was to constitute a real chance to stop the bloodshed. But the armed terrorist groups didn't take it seriously and didn't commit to any of its articles," the military command statement said.
"The armed terrorist groups took advantage of the declared truce system and mobilised terrorists and weapons and regrouped to continue its attacks on civilian and military areas."