(CNN) Syria's nascent ceasefire hung in the balance Monday after an airstrike on a United Nations aid convoy led the US to question Russia's commitment to calming violence in the war-torn country and its ability to influence its ally in Damascus. "What happened today has dealt a serious blow to efforts to bring peace to Syria," said a senior US administration official, who added that the strike "raises very serious questions about whether the Russians can deliver their end of the arrangement" to rein in the Syrian regime. Outrage after aid convoy, warehouse attacked in Syria Asked if the US was still open to extending ceasefire efforts, a second administration official said Russia now bears the burden of demonstrating its commitment to peace efforts, "otherwise, there will be nothing to extend and nothing to salvage."