Russia has accused the United States of failing to fulfil its obligations under the truce agreement in Syria.
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 deliveries to besieged and hard-to-reach areas.
A convoy of 20 lorries carrying desperately needed supplies for rebel-held eastern Aleppo are awaiting clearance at the Turkish border.
Mr de Mistura said the planned withdrawal of both government and rebel forces from the Castello Road, a major artery running around northern Aleppo into the rebel-held east, was also proving complicated.
Under the truce deal, both sides are required to pull back their fighters, heavy weapons and other arms at least 500m (547 yards) away from the road.
Two checkpoints will then be established on the route, initially operated by the Syrian Arab Red Crescent, with security provided by Russian military personnel.
Both government troops and rebels are ready to pull back but will not begin until they see their opponents do the same, according to the UK-based Syrian Observatory for Human Rights.
“Today the withdrawal is supposed to happen, with aid entering tomorrow,” Zakaria Malahifji, of the Aleppo-based rebel group Fastaqim, told Reuters news agency.
“This is what is supposed to happen but there is nothing to give hope,” the rebel spokesman added. “There is great fear because the regime exploits every opportunity.”
The Syrian truce
- A deal was brokered by Russia and the US that began with a nationwide ceasefire from Monday between the armed opposition and the Syrian government, but not jihadist groups
- The ceasefire will be renewed every 48 hours if it holds
- It is meant to allow for “unimpeded and sustained humanitarian access” to besieged areas, including Aleppo
- If the truce holds for a week, Russia and the US will bomb militant groups together, including so-called Islamic State and Jabhat Fateh al-Sham (formerly the Nusra Front)
- The “legitimate opposition” are meant to distance themselves from such groups
- The deal has faced widespread scepticism, not least because the US backs anti-government rebel groups while Moscow is a key ally of the Syrian government
- The deal could pave the way for a political transition, the US says