BEIRUT (Reuters) - 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.

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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.

KERRY’S GAMBLE

The week-old attempt at a ceasefire, negotiated by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, could be the final attempt by U.S. President Barack Obama to negotiate an end to Syria’s civil war.

Kerry called on Moscow to halt Syrian government airstrikes, including on aid convoys, and indicated that the United States had not received official word from Russia that the ceasefire deal was dead.

“The Russians made the agreement. So we need to see what the Russians say,” Kerry said before meeting Saudi Crown Prince Mohammed bin Nayef in New York. “But the point – the important thing is the Russians need to control Assad, who evidently is indiscriminately bombing, including of humanitarian convoys.”

The United Nations said that only Washington and Moscow could declare it over, as they were the ones who originally forged the deal.

The air strikes appeared particularly heavy in insurgent-held areas west of Aleppo, near the rebel stronghold of Idlib province. And in eastern Aleppo, a resident reached by Reuters said there had been dozens of blasts.

“It started with an hour of extremely fierce bombing,” said Besher Hawi, the former spokesman for the opposition’s Aleppo city council. “Now I can hear the sound of helicopters overhead. The last two were barrel bombs,” he said, the sound of an explosion audible in the background.

Abu al-Baraa al-Hamawi, a rebel commander, said the most intense bombardments had taken place in areas west of Aleppo, the same area where the aid convoy was hit. “The regime and Russians are taking revenge on all the areas,” he said.

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Russian and U.S. officials met in Geneva on Monday to try to extend the truce, and the International Syria Support Group - the countries backing the Syria peace process - was scheduled to meet on Tuesday in New York to assess the agreement.

RETURN TO THE BATTLEFIELD

But like the Syrian army, the rebels spoke of returning to the battlefield.

The coordinator of Syria’s main opposition group said on Monday the ceasefire never took hold and called on the world to put an end to the “criminality” of the Syrian government.

“There was no ceasefire to begin with for us to say whether it failed or succeeded,” Riad Hijab, general coordinator of the High Negotiations Committee, told reporters.

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 envisaged under the truce have yet to go in.

The United Nations said it had received government approval to reach nearly all the besieged and hard-to-reach areas where it sought to bring aid, but access to many areas was still constrained by fighting, insecurity and administrative delays.

Already widely violated since it took effect, the ceasefire came under added strain at the weekend when Russia said jets from the U.S.-led coalition against Islamic State killed more than 60 Syrian soldiers in eastern Syria.

Assad called that incident “flagrant aggression”. Washington called it a mistake.

The ceasefire is the second negotiated by Washington and Moscow since Russia joined the war in September 2015. But while it led to a significant reduction in fighting at the outset, violence has increased in recent days and aid has mostly failed to arrive.

Plans to evacuate several hundred rebels from the last opposition-held district of Homs city have also overshadowed the agreement, with rebels saying it would amount to the government declaring the ceasefire over. The Homs governor said the plan had been postponed from Monday to Tuesday.

Washington and Moscow back opposite sides in the war between Assad’s government and the insurgents, while both oppose the Islamic State jihadist group. Russia joined the war a year ago on Assad’s side, tipping it firmly in his favor.