Syrian or Russian aircraft struck an aid convoy near Aleppo on Monday and killed 12 people, according to a war monitor, as the Syrian military declared a one-week truce brokered by the United States and Russia over.

The United Nations confirmed the convoy was hit but gave no details on who carried out the attack or how many died as world leaders converged on New York for their annual U.N. gathering under the shadow of fresh violence in the Syrian civil war.

The Syrian Observatory for Human Rights monitoring group said the attacks were carried out by either Syrian or Russian aircraft, adding that there had been 35 strikes in and around Aleppo since the truce ended.

A humanitarian aid group said the death toll was higher. Fourteen Syrian Arab Red Crescent volunteers were killed, Elhadj As Sy, secretary general of the International Federation of Red Cross and Red Crescent Societies, told a U.N. summit.

At least 18 of 31 trucks in a U.N. and Syrian Arab Red Crescent (SARC) convoy were hit along with an SARC warehouse, said U.N. spokesman Stephane Dujarric. The convoy was delivering aid for 78,000 people in the hard-to-reach town of Urm al-Kubra in Aleppo Governorate, he said.

U.N. aid chief Stephen O'Brien said initial reports indicated many people had been killed or seriously wounded, including SARC volunteers, and that if the "callous attack" was found to be deliberate it amount to a war crime.

"Notification of the convoy ... had been provided to all parties to the conflict and the convoy was clearly marked as humanitarian," he said in a statement, calling for an immediate, independent investigation.

The attack appeared to signal the imminent collapse of the latest effort by the United States and Russia to halt Syria's 5 1/2-year-old civil war.

"We don't know if it can be salvaged," said a senior Obama administration official of the effort by the United States and Russia, which back opposite sides in the conflict.

"At this point the Russians have to demonstrate very quickly their seriousness of purpose because otherwise there will be nothing to extend and nothing to salvage," the official, who spoke to reporters on condition of anonymity, added.

Moscow supports Syrian President Bashar al-Assad with its air force. The Syrian military could not immediately be reached for comment on the attack. But 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".

A local resident told Reuters by phone that the trucks were hit by about five missile strikes while parked in a center belonging to the Syrian Red Crescent in Urm al-Kubra, a town near Aleppo. The head of the center and several others were badly injured.