The U.N.'s emergency relief coordinator said Monday he is "pained and disappointed" that convoys have yet to be allowed into Aleppo where as many as 275,000 people are in need of food, water and medical aid.
Humanitarian deliveries to Aleppo were one prong of a cease-fire deal reached between the United States and Russia that was due to end late Sunday.
There was no official declaration of its fate by early Monday, but with deadly airstrikes on rebel positions in Aleppo Sunday and one by the U.S.-led coalition Saturday that killed dozens of Syrian soldiers, the pact appeared to be unraveling.
U.N. Under-Secretary-General for Humanitarian Affairs Stephen O'Brien reiterated the world body's demands that it be allowed unimpeded access to deliver supplies and evacuate those in need of critical medical attention.
"The people of Syria have suffered long enough. Millions of Syrian civilians continue to face horrific deprivation and violence, especially those trapped in besieged and hard-to-reach areas," O'Brien said in a statement.
The U.S. and Russia have influence on the conflict from their support of competing sides, and in recent days tensions between them have risen as the cease-fire appeared shaky.
U.S. Secretary of State John Kerry on Sunday blamed Syrian President Bashar al-Assad's government for blocking aid deliveries and said Russia must do more to put pressure on him.
"So let me just say this clearly: Russia signed up to a cessation of hostilities. Assad said he would live by it," Kerry said. "Then he needs to stop and let the joint implementation center get set up so Russia and the United States can coordinate in order to avoid the kind of terrible thing that happened yesterday that we all acknowledge and regret."
The U.S. military said it may have unintentionally hit the Syrian soldiers while targeting Islamic State fighters in eastern Syria.
The Russian Defense Ministry said U.S. jets killed more than 60 Syrian soldiers in the city of Deir al-Zor in four air strikes by two F16 and two A10 fighter jets coming from the direction of Iraq. Moscow called for an emergency meeting of the U.N. Security Council to address the strikes.
U.S. Ambassador Samantha Power said Moscow's request for the special meeting was a "stunt," while her Russian counterpart Vitaly Churkin accused the U.S. of violating agreements that it would not target army positions.
Churkin called the strike a "bad omen" for the U.S.-Russia deal to halt Syria's war, which has killed hundreds of thousands of people since it erupted in 2011.