Syria's ceasefire "will not hold out", a senior rebel official in Aleppo warned on Saturday, as air strikes and shelling continued in some places and promised aid deliveries failed to come through.

The ceasefire is the result of an agreement between Russia, which backs Syrian President Bashar al-Assad with air power, and the United States, which supports some rebel groups, and has cooled fighting since coming into effect last Monday.

On Friday, Washington and Moscow agreed to extend the ceasefire.

Insurgents say they only reluctantly accepted the initial deal, which they believe is skewed against them, because it could relieve the dire humanitarian situation in besieged areas they control, including in eastern Aleppo.

"The truce, as we have warned, and we told the (U.S.) State Department—will not hold out," the rebel official said, pointing to the continued presence of a U.N. aid convoy at the Turkish border awaiting permission to travel to Aleppo.

"It is not possible for the party (Russia) that wages war against a people to strive to achieve a truce, as it is also not possible for it to be a sponsor of this agreement while it bombs night and day, while on the other side, the other party—America—has the role of spectator," he said.

Moscow has itself accused rebels of breaking the truce and said Washington needs to do more to make them abide by its terms, including separating from the jihadist Jabhat Fateh al-Sham, which only broke formal allegiance to al Qaeda in July.

Both sides have accused the other of being responsible for aid deliveries being stuck far from Aleppo, where army and rebel forces were supposed to retire from the Castello Road which leads into besieged, insurgent-held eastern districts.

Russia on Friday said the Syrian army had initially withdrawn but returned to its positions after being fired on by rebels, who in turn say they saw no sign of government forces ever leaving their positions.

The United Nations pointed the finger at the government for holding up aid by denying letters guaranteeing access.