Monitors say warplanes hit a Syrian convoy bringing desperately needed aid to a hard-to-reach town, as Syria's military declared an end to the already fraying a week-old cease-fire.
The United Nations confirmed the convoy was hit on September 19, but it was unclear whose aircraft were behind the strike that Syrian activists said killed at least 12 people.
The Syrian Observatory for Human Rights monitoring group said the attacks were carried out by either Syrian or Russian aircraft. The group said that there had been 35 strikes in and around Aleppo since the truce ended.
U.S. administration officials also confirmed the convoy had been hit, calling it a serious blow to the truce that was brokered by Washington and Moscow and took effect on September 12.
In a statement issued on September 19, the Syrian military called an end to the truce, saying "armed terrorist groups" had repeatedly violated it. It accused the groups of using the cease-fire to mobilize and arm themselves while attacking government-held areas.
in Washington, State Department spokesman John Kirby called on Russia to clarify the statement by Syria, which Moscow has sought to bolster with an air campaign that began a year ago.
"We will be consulting with our Russian counterparts to continue to urge them to use their influence on [President Bashar al-Assad] to these ends," Kirby said in a September 19 statement.
"While we have seen comments attributed to the Syrian military, our arrangement is with Russia, which is responsible for the Syrian regime's compliance, so we expect Russia to clarify their position."
UN spokesman Stephane Dujarric said at least 18 of 31 trucks in a UN and Syrian Arab Red Crescent convoy were hit along with a Red Crescent warehouse.
The convoy was delivering aid for 78,000 people in the hard-to-reach town of Urm al-Kubra in Aleppo Governorate, he said.
The cease-fire is the second negotiated by Washington and Moscow since Russia joined the war in September 2015. But while it led to a significant reduction in fighting at the outset, violence has increased in recent days and aid has mostly failed to arrive.
The cease-fire came under added strain over the weekend when jets from the U.S.-led coalition killed dozens of soldiers that Moscow said were Syrian troops.
Washington called the attack a mistake.