The convoy from the UN and the Syrian Arab Red Crescent (SARC) was en route to the hard-to-reach town of Orum al-Kubra, in Aleppo province, to deliver humanitarian assistance to 78,000 people, the UN said.
The Syrian Observatory for Human Rights said 12 aid workers and drivers were killed in the strike, but the United Nations did not confirm the casualties.
"Our outrage at this attack is enormous," the UN envoy for Syria, Staffan de Mistura, told reporters.
"The convoy was the outcome of a long process of permission and preparations to assist isolated civilians."
Jan Egeland, head of the UN humanitarian task force for Syria, tweeted that there were "many killed and injured" in the strike.
The convoy was "bombed today in spite of de-confliction with parties" in which the aid agencies coordinate their movements with all sides on the ground, he added.
UN aid chief Stephen O'Brien said he was "deeply concerned" by the incident and called on "all parties to the conflict, once again, to take all necessary measures to protect humanitarian actors, civilians, and civilian infrastructure as required by international humanitarian law."
A SARC warehouse was also hit, UN spokesman Stephane Dujarric said.
"Multiple sources have confirmed that the town was shelled this evening," he said.
"However, the situation and reporting on it continues to evolve and we are unable at this time to independently verify casualties," he added.
A member of the team describes the damage after an airstrike, in Aleppo, Syria, Monday, Sept. 19, 2016 (Syrian Civil Defence White Helmets via AP) Source: Syrian Civil Defence White Helmets
The Syrian observatory said President Bashar al-Assad's air force carried out more than 40 air raids since the army announced the end of the ceasefire, killing 36 people.
French Foreign Minister Jean-Marc Ayrault strongly condemned the strike, saying the destruction of the aid convoy underscored "the urgency of a cessation of hostilities in Syria."
The International Committee of the Red Cross (ICRC) confirmed the attack on the convoy.
"The situation on the spot is very chaotic and we are deeply shocked that humanitarian workers and missions have yet again suffered from the brutality of this conflict," ICRC spokeswoman Ingy Sedky told AFP.
Syria's military earlier announced the end of a ceasefire brokered by the United States and Russia, accusing rebels of more than 300 violations and failing to "commit to a single element" of the US-Russia deal.
Source: AFP
Syria calls ceasefire end
Syria's military declared a week-long ceasefire over with air raids reported in Aleppo as US and Russian officials met in Geneva to try to extend the truce.
US Secretary of State John Kerry said it was too early to call the ceasefire finished and the United Nations said only Washington and Moscow could declare it over, as they were the ones who originally agreed it.
Washington said it was working to extend the truce but called on Russia to first clarify the Syrian army's statement that it was over.
Russian and US officials met in Geneva on Monday and the International Syria Support Group - the countries backing the Syria peace process - were scheduled to meet on Tuesday in New York to assess the ceasefire agreement.
But both the Syrian army and the rebels spoke openly of returning to the battlefield.
Syria's army said the seven-day truce period had ended.
It accused "terrorist groups", a term the government uses for all insurgents, of exploiting the calm to rearm while violating the ceasefire 300 times and vowed to "continue fulfilling its national duties in fighting terrorism in order to bring back security and stability".
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Asked about the army's statement, Kerry told reporters in New York the seven days of calm and aid deliveries envisaged in the truce had not yet taken place.
"It would be good if they didn't talk first to the press but if they talked to the people who are actually negotiating this," Kerry said.
"We just began today to see real movement of humanitarian goods and let's see where we are. We're happy to have a conversation with them."
Aid was delivered to the besieged town of Talbiseh in Homs province on Monday, the Red Cross said, for the first time since July. The convoy brought in food, water and hygiene supplies for up to 84,000 people, it said.
But most aid shipments envisioned under the truce have yet to go in, especially a convoy destined for rebel-held eastern parts of Aleppo, where some 275,000 civilians are believed trapped without access to food or medical supplies.
Already widely violated since it took effect, the ceasefire came under added strain at the weekend when Russia said jets from the US-led coalition against Islamic State killed more than 60 Syrian soldiers in eastern Syria.
Assad called that incident "flagrant aggression". Washington has called it a mistake.
