Russia has accused the United States of failing to fulfil its obligations under the truce agreement in Syria.
A Russian defense 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.