A defence ministry statement said Washington was using a “verbal curtain” to hide its reluctance to rein in the rebel groups it supports.
The truce has broadly held since taking effect on Monday although the Russian-backed Syrian army and rebels have accused each other of many violations.
Meanwhile, the UN has warned there is a “problem” with getting aid into Syria.
Special envoy Staffan de Mistura placed responsibility on the Syrian government which, he said, had not yet provided the “facilitation letters” that would allow aid convoys to pass through army checkpoints and reach besieged areas.
“We cannot let days of this reduction of violence be wasted by not moving forward,” he told reporters in Geneva.
The Russian defence ministry insisted that from the very beginning of the truce, Moscow had been fulfilling its obligations, which includes ensuring that the Syrian air force does not bomb areas held by mainstream rebel forces and setting up checkpoints in divided second city of Aleppo.
It therefore found “confusing” recent comments by US officials that expressed doubts about whether Russia would be able to deliver.
“Only the Syrian army has been observing the ceasefire regime… while the US-led ‘moderate opposition’ has been increasing the number of shellings of residential quarters,” the ministry statement said.
“Moreover, it appears that the ‘verbal curtain’ of Washington is aimed at hiding the non-fulfilment of the US obligations.”
The US, it added, had not yet compelled rebels to separate themselves on the ground from allied fighters from Jabhat Fateh al-Sham, a jihadist group known as al-Nusra Front until it formally broke off ties with al-Qaeda in July.
The move is necessary before the US and Russia can start conducting joint air strikes targeting Jabhat Fateh al-Sham and so-called Islamic State (IS). -BBC