NEW YORK — Aghast at a deadly aerial bombing of 31 humanitarian supply trucks authorized to travel in Syria, United Nations officials suspended all aid convoys in the war-ravaged country Tuesday, described the attack as a possible war crime, and called the bombers cowards.

The strike on the trucks, which were carrying critically needed supplies of food and medicine bound for rebel-held areas of Syria’s western Aleppo province, happened Monday evening after the Syrian military declared that it regarded a seven-day partial cease-fire as over.

The convoy, escorted by the Syrian Arab Red Crescent, was among the first to try to deliver humanitarian aid to the rebel-held areas under the cease-fire agreement. Members of the group said its local chief, Omar Barakat, was among at least 12 people killed; UN officials in Geneva said the death toll was unclear.


The International Committee of the Red Cross said in a statement that about 20 civilians were killed and much of the aid destroyed. Peter Maurer, the organization’s president, called the attack a “flagrant violation” of international law.

Both Syria and Russia, its principal ally in the conflict, denied responsibility. The US Central Command said the attack had not been carried out by US military aircraft, Reuters reported.

The head of the UN agency that coordinates aid, Stephen O’Brien, said the attack would amount to a war crime if it were found to have targeted humanitarian aid workers. He called for an independent investigation.

Witnesses said multiple strikes had hit the convoy as workers were unloading aid, and then hit rescue workers who arrived to help.

Eighteen of the convoy’s 31 trucks — which UN officials say were clearly marked and were carrying wheat flour, 9 tons of medicine, and clothing for about 78,000 people — were destroyed. Benoit Carpentier, a spokesman for the International Federation of Red Cross and Red Crescent Societies, said a hospital had also been destroyed.


Aid convoys have endured sniper fire and shelling during the five years of the Syrian conflict, but Monday’s attack is believed to be the first time one was hit by an airstrike.

The convoy attack and the declaration by the military were the strongest signs yet of the gradual unraveling of a broader agreement between Russia and the United States aimed at restarting peace talks to end the conflict in Syria, which has killed an estimated 500,000 people and displaced millions.

The New York Times chronicled the seven-day cease-fire negotiated by Washington and Moscow — which back opposite sides in the conflict — through the observations of Syrians around the country. By Day 5, it was clear the agreement was unraveling.

John Kirby, spokesman for the State Department, called Monday’s attack an “egregious violation” of the agreement, but on Tuesday, Secretary of State John Kerry declared: “The cease-fire is not dead.”

Kerry met with his Russian counterpart, Sergey V. Lavrov, and other diplomats representing the International Syria Support Group in New York Tuesday morning as the United Nations General Assembly got underway. Further meetings of the support group, a 17-nation effort to halt the conflict led by Kerry and Lavrov, are planned this week.

“The mood of the meeting was very much that nobody wants to give this thing up,” said Boris Johnson, the British foreign secretary. “Quite frankly, the Kerry-Lavrov process is the only show in town, and we’ve got to get that show back on the road.”


France’s foreign minister, Jean-Marc Ayrault, was less optimistic. He said the effort by Kerry and Lavrov had “reached its limits.” Ayrault said he had suggested at the meeting a way to monitor any cease-fire to “verify that on the ground it is being respected.”

Secretary-General Ban Ki-moon, in his opening speech to the General Assembly Tuesday, denounced the convoy bombing, his voice tense with anger.

“Just when we think it cannot get any worse, the bar of depravity sinks lower,” Ban said in the speech, his last as leader of the organization after a decade in the job, much of it preoccupied with Syria.

“The humanitarians delivering lifesaving aid were heroes,” he added. “Those who bombed them were cowards.”