Secretary of State John Kerry and senior officials from two dozen nations meeting here Tuesday declared that Syria's cease-fire "is not dead " but offered no ideas on how it can be preserved after heavy fighting broke out again, including the bombing of a humanitarian aid convoy by what Pentagon officials said was likely a Russian jet.

"The mood of the meeting is that nobody wants to give this thing up. It's the only show in town," British Foreign Secretary Boris Johnson said after an hour-long session that others described as "tense" and "dramatic."

Pentagon officials, who earlier had said they were uncertain who was responsible for the Monday airstrike, which took place as aid was being offloaded from trucks west of Aleppo, said that their "preliminary" assessment was that a Russian Su-24 operating "overhead at the time" was responsible for the bombing.

The officials said they had tracked the aircraft and were certain it was Russian and did not belong to the Syrian air force, which uses the same planes. Pentagon officials spoke on the condition of anonymity to discuss internal assessments.

Ben Rhodes, President Barack Obama's deputy national security adviser, said Tuesday night, "We hold the Russian government responsible for airstrikes in this airspace given their commitment under the cessation of hostilities was to ground air operations where humanitarian assistance was flowing."

Russia has said its own assessments show that no Russian or Syrian aircraft were responsible for the bombing, which left at least 20 civilians dead, including the head of the local Syrian Red Crescent, and suggested it was the result of a ground attack by al-Qaida-linked forces against the Syrian military.

The Russian Foreign Ministry said, "Moscow is outraged by the attempts to blame Russia and Syria for the shelling of a humanitarian convoy in Syria."

"The Russian military are making checks, including by using objective-monitoring systems, to clarify all details of the incident in Urum al-Kubra on the night of September 19," the ministry said in a statement on its website late Tuesday. "In the meantime, we can state responsibly that no airstrikes were carried out by either Russia or Syria against the U.N. humanitarian convoy on the southwestern edge of Aleppo."

Maj. Gen. Igor Konashenkov, a Russian Defense Ministry spokesman, earlier told reporters that a Russian drone that had been tracking the convoy showed that it was accompanied by a rebel "pickup truck mounted with a large-caliber mortar," implying that the aid convoy had served as cover for a rebel operation.

In the wake of the convoy bombing, the United Nations and other relief agencies suspended all aid shipments across combat lines. The deadly incident capped a rapid unraveling of week-old truce efforts brokered by the United States and Russia. The plan was intended to open routes to aid thousands of besieged Syrians, possibly spur greater military counterterrorism cooperation between Moscow and Washington, and create conditions for a resumption of negotiations on a long-term political solution to Syria's civil war.

What was left Tuesday was a return to shelling and airstrikes in places that had hoped to receive critical food and medicine, and an escalation of hostility between the United States and Russia as the two governments presented starkly different versions of what transpired the day before.

Kerry planned to meet Tuesday night with Russian Foreign Minister Sergei Lavrov and propose to him that the only way to restart the truce and prove Moscow's commitment and influence was the immediate grounding of the Syrian air force, which has dropped barrel bombs and chemical weapons on both the opposition and civilians.

The dispute took place as more than 100 world leaders have gathered here for the annual U.N. General Assembly this week. On Wednesday morning, Kerry is expected to take the U.S. chair at a special U.N. Security Council meeting on Syria.

A spokesman for Russian President Vladimir Putin, who is not attending the U.N. meeting, described hopes for resumption of the cease-fire as "weak." Conditions for resuming the truce, spokesman Dmitry Peskov said in Moscow, were "quite simple: [The militants] must stop firing. The terrorists must stop attacking the armed forces of Syria. And certainly, it would be nice if our American colleagues didn't accidentally bombard Syrians."

The reference was to a bombing attack Saturday in which coalition aircraft, apparently targeting Islamic State forces in eastern Syria, instead bombed a Syrian military unit. Russia and Syria said at least 62 soldiers were killed and more than 100 were wounded in the strike, which the U.S. military has said it is investigating.

Kerry held a brief, private meeting with Lavrov before both attended the larger gathering with foreign ministers from Europe and the region that are partners in the U.S.-led coalition, as well as Iran, which is supporting Syrian President Bashar Assad along with Russia. Lavrov left the group meeting without comment.

U.N. envoy Staffan de Mistura, who also attended the session, said afterward: "The cease-fire is in danger. The cease-fire has been seriously affected. But the only ones who can announce the cease-fire is dead are [the United States and Russia], and they have today not done so. They want to give it another chance."

But French Foreign Minister Jean-Marc Ayrault told reporters that "the U.S.-Russia negotiation has reached its limits" and that they "alone will not be able to achieve this cease-fire." Ayrault said he had proposed a new mechanism for control and monitoring the cease-fire on the ground. The group plans to meet again this week.

In his major U.N. speech Tuesday, Obama made only a brief mention of Syria, saying that "there is no ultimate military victory to be won. We're going to have to pursue the hard work of diplomacy that aims to stop the violence and deliver aid to those in need."

Obama has been supportive of Kerry's diplomatic efforts with Russia, over the objections of the Pentagon, which mistrusts Moscow's intentions.

U.N. Emergency Relief Coordinator Stephen O'Brien said that "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."

It the attack proved to have been intentional, he said, "it would amount to a war crime."

The attack 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 unloaded supply trucks, the Syrian aid group and the International Committee of the Red Cross 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, Syrian civil-defense volunteer Ammar al-Selmo spoke in English in front of burning warehouses. He held up diapers and blankets supplied by the U.N. refugee agency. Selmo, the Aleppo director of the White Helmets, a volunteer civil-defense force in rebel-held areas, said Tuesday in an interview that "the place turned into hell, and fighter jets were in the sky." 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.

"It was pure chaos," said Syrian medic Bakry Ebeid, a friend and colleague of Barakat's. 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.

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."