Updated

Syria's cease-fire has faltered further after an aid convoy was hit by airstrikes, with activists saying at least 12 people were killed in the attack, mostly truck drivers and Red Crescent workers.

The strikes late on Monday came just hours after the Syrian military declared the week-long U.S.-Russian brokered cease-fire had failed. The United States said it was prepared to extend the truce deal and Russia — after blaming rebels for the violations — suggested it could still be salvaged.

It was not clear who was behind the attack, which sent a red fireball into the sky in the dead of night over a rural area in Aleppo province. Both Syrian and Russian aircraft operate over Syria, as well as the U.S.-led coalition that is targeting the Islamic State group.

U.N. officials said the U.N. and Red Crescent convoy was delivering assistance for 78,000 people in the town of Uram al-Kubra, west of the northern city of Aleppo.

Initial estimates indicate that about 18 of the 31 trucks in the convoy were hit, as well as the Red Crescent warehouse in the area.

The Britain-based Syrian Observatory for Human Rights, an activist group that tracks the civil war, said at least 12 were killed in the attack, mostly truck drivers and Red Crescent workers. The Syrian Civil Defense, the volunteer first responder group also known as the White Helmets, confirmed that casualty figure.

Jan Egeland, humanitarian aid coordinator in the office of the U.N. envoy for Syria, told The Associated Press in a text message that the convoy was "bombarded."

Egeland added, "It is outrageous that it was hit while offloading at warehouses."

U. N. Humanitarian Chief Stephen O'Brien called on "all parties to the conflict, once again, to take all necessary measures to protect humanitarian actors, civilians, and civilian infrastructure as required by international humanitarian law."

The convoy, part of a routine interagency dispatch operated by the Syrian Red Crescent, was hit in rural western Aleppo province. The White Helmets first responder group posted images of a number of vehicles on fire and a video of the attack showed huge balls of fire in a pitch black area, as ambulances arrive on the scene.

A Red Crescent official in Syria confirmed the attack, but said no further information was available.

Elsewhere on Monday, at least 20 civilians, including a 1-year-old girl, were killed in fresh airstrikes on rebel-held Aleppo city and the surrounding areas, according to the Observatory. And Russia said government positions in southwestern Aleppo came under attack from militant groups, including a massive barrage of rockets.

With the week-old cease-fire under threat, both Moscow and Washington indicated a desire to try and salvage the agreement that had brought a brief respite to at least some parts the war-torn country.

In the wake of the Syrian military declaration, U.S. Secretary of State John Kerry acknowledged that the first stage of the truce — which called for a week of calm and the delivery of humanitarian aid to several besieged communities — had never really come to fruition. Earlier in the day, Kerry told reporters on the sidelines of the U.N. General Assembly that the truce was "holding but fragile."

The State Department said that it was ready to work with Russia to strengthen the terms of the agreement and expand deliveries of humanitarian aid. Spokesman John Kirby said Russia, which is responsible for ensuring Syria's compliance, should clarify the Syrian position.

A Russian Foreign Ministry statement late Monday night appeared to signal that the deal could still be salvaged, saying that the failure by the rebels in Syria to respect the cease-fire threatens to thwart the agreement.

The cease-fire came into effect on Sept. 12. Under terms of the agreement, the successful completion of seven days of calm and humanitarian aid deliveries would be followed by an ambitious second-stage plan to set up a joint U.S.-Russian coordination center to plan military strikes against the Islamic State group and a powerful al-Qaida-linked militant faction.

But from the start, the truce has been beset by difficulties and mutual accusations of violations.

Aid deliveries to the besieged eastern districts of Aleppo have not reached their destination. The U.N. accused the government of obstructing the delivery while Russian officials said rebels opened fire at the delivery roads.

Rebel forces and activists say government planes have bombed areas that are under the truce agreement, including rebel-held parts of Aleppo. At least 22 civilians were killed in government bombings over the last week, according to the Observatory, an opposition activist group. It said four civilians were killed in government-held areas. There were no independent reports of deaths of civilians on the government-side since the cease-fire came into effect.