The United Nations and other relief agencies suspended all humanitarian convoys across combat lines in Syria on Tuesday after a bomb attack on an aid shipment killed more than 20 people near Aleppo as a cease-fire crumbled.

The convoy’s deadly fate Monday capped a rapid unraveling of week-old truce efforts, brokered by the United States and Russia. The plan intended to open routes to aid thousands of besieged Syrians and possibly spur greater military cooperation between Moscow and Washington to battle militants such as the Islamic State.

What was left Tuesday was a return of shelling and airstrikes in places that had hoped to receive critical food and medicine — including embattled Aleppo — and angry denials from Russia that it was responsible for targeting the aid convoy.

The International Committee of the Red Cross called Monday’s attack a “flagrant violation of international law,” and the U.N. said it could amount to a war crime. The convoy was carrying U.N. supplies distributed by the Syrian Arab Red Crescent in the village of Urum al-Kubra near Aleppo.

“Notification of the convoy — which planned to reach some 78,000 people — had been provided to all parties to the conflict, and the convoy was clearly marked as humanitarian,” said U.N. Emergency Relief Coordinator Stephen O’Brien.

If the attack deliberately targeted humanitarian workers, O’Brien said, “it would amount to a war crime.”

It sent a massive fireball into the sky over rural Aleppo, killing “around 20 civilians” and Syrian Arab Red Crescent sub-branch director Omar Barakat as they unload supply trucks, the Syrian aid group and ICRC said in a joint statement.

"The attack deprives thousands of civilians of much-needed food and medical assistance," the statement added.

In a video recorded Monday night, a Syrian civil defense volunteer spoke in English in front of the burning warehouses. He held up diapers and blankets supplied by the U.N. refugee agency.

"The place turned into hell, and fighter jets were in the sky," said Ammar al-Selmo, the Aleppo director of the White Helmets, a volunteer civil defense force in rebel-held areas. The group has headquarters less than a mile from where the convoy was hit.

Elsewhere, other non-food items such as vitamin C and cream to treat burns had been blown out of their boxes and were scattered on the warehouse floor.

In this video shared by the Syrian Civil Defense, a rescue worker walks through the aftermath of an airstrike on an aid convoy near Aleppo which killed at least 12 people. (YouTube/Syrian Civil Defense)

"It was pure chaos," Syrian medic Bakry Ebeid, a friend and colleague of Barakat. Those aid workers who survived the strike attempted to treat others who had been severely injured.

"But for some, like Omar [Barakat], it was too late," Ebeid said.

In Moscow, Russia’s Defense Minister strongly denied suggestions — carried by some Syrian activists — that its warplanes hit the convoy.

Maj. Gen. Igor Konashenkov, a Defense Ministry spokesman, did not directly accuse rebel factions opposing the government of Syrian President Bashar al-Assad, a key Russian ally. But he said the fire in the aid convoy “strangely began” during a “massive rebel offensive” in the northern city of Aleppo.

The United States supports some opposition groups seeking to topple Assad. Both sides now blame the other for collapsing the cease-fire.

The strike on the convoy "raises very serious questions about whether the Russians can deliver," a senior administration official said in a briefing with reporters Monday.

“From all indications, it was an airstrike, and it wasn't one from the coalition," said a second U.S. official who also participated in the briefing. "We don't know at this point whether it was the Russians or the regime."

Meanwhile, U.N. and International Red Cross missions to villages in various parts of Syria have been suspended, officials said.

"There were planned convoys today, and those are not happening," said David Swanson, a spokesman for the U.N. Office for the Coordination of Humanitarian Affairs. He said the pause was needed to “reassess and revaluate the situation on the ground."

Kremlin spokesman Dmitry Peskov told reporters that there was “little hope” of renewing the cease-fire, blaming the Americans for not separating “terrorists” who were not part of the truce from the moderate opposition fighters backed by Washington and others.

The deal had envisioned ­eventual coordination between Russia and the United States of counterterrorism airstrikes against the Islamic State and a former al-Qaeda affiliate, now known as Jabhat Fatah al-Sham.

The Russian government announced that group launched a broad-based offensive against the government-controlled southwestern edge of Aleppo on Monday night before being repelled.

The report could not be verified, but is further evidence of a return to heavy fighting in the area after the week-long lull that brought a fleeting respite from the five-year conflict.

With the cease-fire in tatters, diplomatic efforts shifted to crisis mode at the annual U.N. General Assembly in New York.

Secretary of State John F. Kerry met early this morning with Russian Foreign Minister Sergei Lavrov. The two made no comment afterwards, and headed immediately into a larger meeting of the Syrian support group that includes the United States and its partners backing the Syrian rebels, and Russia and Iran, which support Assad.

Tensions were already at a high between the United States and Russia. Over the weekend, coalition warplanes, apparently inadvertently, struck a camp of Syrian government troops in the eastern part of the country, killing what Syria said were more than 60 soldiers. The U.S. military quickly acknowledged the strike, saying it was targeting Islamic State positions. It offered its regret and said it was investigating what appeared to have been an intelligence failure.

DeYoung reported from New York and Roth from Moscow. Zakaria Zakaria in Istanbul, and Heba Habib in Berlin also contributed to this report.