BEIRUT — A U.N. humanitarian aid convoy inside Syria was hit by airstrikes, U.N. officials said, as the Syrian military declared Monday that the week-long U.S.-Russian brokered cease-fire had failed.

With the truce apparently teetering on the brink of collapse, the U.S. said it’s prepared to extend the agreement, and Russia – after blaming rebels for the violations – suggested it could still be salvaged.

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 Aleppo. Initial estimates indicate that at least 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 said at least 12 people 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, said 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 western Aleppo province. The White Helmets first responder group posted images of a number of vehicles on fire at night. A video of the attack showed huge balls of fire as ambulances arrived on the scene.

Elsewhere at least 20 civilians were killed in new airstrikes on rebel-held Aleppo 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 in danger of unraveling, both Moscow and Washington have indicated a desire to try to salvage the agreement – which 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 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.

Earlier Monday, Lt. Gen. Sergei Rudskoi of the Russian military’s General Staff said in a briefing that Damascus had fulfilled its obligations.

The current tensions come on the heels of the weekend air strike by the U.S.-led coalition on Syrian army positions near Deir el-Zour. Syria and Russia blasted Washington over the attack.