At least 18 trucks in the 31-vehicle convoy were hit
All aid convoys in Syria have been suspended after a deadly attack that hit at least 18 trucks carrying supplies, the UN has said.
The organisation claimed the attack in rebel-held Urm al Kubra could amount to a war crime if it was deliberate.
A spokesman for the United Nations humanitarian aid agency said the temporary suspension of aid would stay in force while the security situation was reviewed in Syria.
He added that it had been a "very, very dark day" for aid agencies around the world.
Reports say least 12 aid workers and drivers in the 31-vehicle convoy operated by the Syrian Arab Red Crescent (SARC) were killed.
The victims included SARC sub-station director Omar Barakat, described by the organisation as a "brave member of our family of committed staff and volunteers, working relentlessly to alleviate the suffering of the Syrian people".
"It is totally unacceptable that our staff and volunteers continue to pay such a high price because of the ongoing fighting," added SARC president Dr Abdulrahman Attar.
The attack late on Monday happened just hours after Syria's military declared an end to the week-long ceasefire brokered by the US and Russia.
The British-based Syrian Observatory for Human Rights (SOHR) said it was carried out by either the Syrian government or the Russian military.
It comes as ministers from the 23-nation International Syria Support Group (ISSG) meet alongside the UN General Assembly in New York to decide which steps to take next to secure peace.
UN Secretary-General Ban Ki Moon opened the debate with a stinging attack on the Syrian government, saying it had killed the most civilians during the country's five-year civil war.
In his final address to the annual gathering of world leaders at the UN, he said "powerful patrons that keep feeding the war machine also have blood on their hands."
He added the Syrian government "continues to barrel bomb neighbourhoods and systematically torture thousands of detainees".
Mr Ban told the meeting: ""Present in this hall today are representatives of governments that have ignored, facilitated, funded, participated in or even planned and carried out atrocities inflicted by all sides of the Syria conflict against Syrian civilians."
As well as the aid convoy attack, SOHR said another 20 people were believed to have been killed in separate incidents as the ceasefire ended.
UN aid chief Stephen O'Brien called for an investigation into the strike.
"Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," he said.
The US also expressed its outrage, with officials saying the attack could only have been carried out by the Syrian regime or Russia.
State Department spokesman John Kirby said: "The destination of this convoy was known to the Syrian regime and the Russian Federation.
"And yet these aid workers were killed in their attempt to provide relief to the Syrian people.
"Given the egregious violation of the cessation of hostilities we will reassess the future prospects for cooperation with Russia."
Both Russia and Syria have denied carrying out the attack.
In a statement read out on Russian state TV, a ministry spokesman suggested it was the work of militants.
"All information on the whereabouts of the convoy was available only to the militants controlling these areas," he said.
The Syrian state news agency SANA said: "There is no truth to reports carried by some media outlets that the Syrian Arab Army targeted a humanitarian aid convoy in the Aleppo countryside."
US Secretary of State John Kerry has been meeting his Russian counterpart Sergei Lavrov in New York to discuss the latest developments in Syria, it has been reported. Both sides have suggested the truce deal could be salvaged.
The ceasefire had already come under pressure following a series of attacks at the weekend and a spat between Moscow and Washington over US-led airstrikes which killed around 60 Syrian soldiers.
America apologised for the bombing, which occurred on a base near Deir al Zor airport, insisting that its intended target was Islamic State fighters.
Syria's government has accused rebels of more than 300 violations and failing to "commit to a single element" of the ceasefire.