A week into a cease-fire in Syria brokered by Russia and the US, Russian-made warplanes hit a UN aid convoy traveling to the besieged town of Aleppo to provide relief to Syrians at the scene of some of the most intense fighting in the country's five-year civil war.

At least 18 of 31 trucks in a UN and Syrian Arab Red Crescent convoy were hit and 12 people were killed, according to Reuters.

UN Secretary General Ban Ki-moon called the airstrike "sickening" in remarks on Tuesday at the UN General Assembly in New York.

"Present in this hall today are representatives of governments that have ignored, facilitated, funded, participated in or even planned and carried out atrocities inflicted by all sides of the Syria conflict against Syrian civilians," Ban said.

"It started with an hour of extremely fierce bombing," Besher Hawi, the former spokesman for the opposition's Aleppo city council, told Reuters of the air raid. "Now I can hear the sound of helicopters overhead. The last two were barrel bombs."

Because the Syrian regime, which has been linked to chemical warfare against civilians and other war crimes, flies Russian-made jets, it can be hard for ground forces distinguish them from the Russian air force, which also carries out airstrikes in Syria, reportedly sometimes on hospitals or with banned munitions.

Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights told The Associated Press that the Syrian air force "does not have the capabilities to carry out such airstrikes within two hours." Abdurrahman added that "it was mostly Russian warplanes who carried out the air raid."

Future of US-Russian relations

President Barack Obama shakes hands with Russia's President Vladimir Putin. REUTERS/Jonathan Ernst

"The United States is outraged by reports that a humanitarian aid convoy was bombed near Aleppo today," State Department spokesman John Kirby said. "The United States will raise this issue directly with Russia. Given the egregious violation of the cessation of hostilities we will reassess the future prospects for cooperation with Russia."

Secretary of State John Kerry had described the cooperation between the US and Russia as possibly "the last chance we have to save a united Syria," and that a truce between the two powers leading to negotiations to end the war "is the only realistic possible solution."

But the Syrian military has declared the cease-fire over, an official failure that comes to a close after the US mistakenly killed more than 60 Syrian soldiers in an airstrike, and the aid convoys Russia promised to let through to Aleppo were bombed.

While the US can claim it had faulty information on the position of the Syrian troops, the position of the UN aid convoy was known to all parties and clearly marked as a humanitarian effort.

A senior US official told Reuters he was not sure if US-Russian relations could be salvaged at this point, as the countries back opposite sides in the Syrian conflict, and their latest attempt at cooperation proved disastrous.

"At this point the Russians have to demonstrate very quickly their seriousness of purpose because otherwise there will be nothing to extend and nothing to salvage," the official told Reuters.