Aid is not reaching some areas due to a lack of permits
Russia has accused the US of failing to uphold its end of the Syria ceasefire agreement, while criticising officials in Washington for voicing scepticism over cooperation with Moscow.
Russia military spokesman Igor Konashenkov hit out at the US for what he called "rhetorical fog" intended "to hide the fact that it is not fulfilling its part of the obligations".
In a statement, he defined Washington's role as "first and foremost to separate 'moderate opposition' groups from terrorists".
"As of the third day (of the truce), only the Syrian army is observing the regime of silence. At the same time, the 'moderate opposition' led by the US is increasing the amount of attacks on residential districts," Mr Konashenkov said.
He insisted: "Russia has respected its obligations to fulfil the ceasefire regime in Syria from the first minute."
US officials have voiced doubts Russia will meet its obligations as part of the agreement struck last week between the two former Cold War adversaries after years of bad blood over Russia's policies over Ukraine and Syria.
Under the ceasefire, the US and Russia said they would set up a "Joint Implementation Centre" to share targeting information for airstrikes in Syria if the truce that took effect on Monday holds for one week.
Several Pentagon officials have told the AFP news agency of their deep unease over the truce, with one saying: "The timeframe is short, but there's a gulf of trust that's larger than the timeframe."
State department spokesman Mark Toner said on Wednesday that "we've seen violations on both sides" in Syria, although the ceasefire is broadly holding.
The United Nations Special Envoy for Syria, Staffan de Mistura, said on Thursday that the country's government was holding up the delivery of UN aid that was meant to move unhindered as part of the peace deal.
"The government, I repeat the government, was expected to provide...permits, authorisations," Mr de Mistura told reporters in Geneva.
They "have not been received", he added.
The lack of permission was "a very major disappointment" even for Syria's ally Russia, Mr de Mistura said.
He added that while the reduction in violence since Monday had been substantial, expecting a "cessation of hostilities" was perhaps ambitious after a war of five years.
Mr de Mistura's humanitarian adviser Jan Egeland said there had been no reports of civilian killings in the past 24 hours and attacks on schools and hospitals had stopped.
Mr Egeland said aid convoys could reach besieged areas like Moadamiyah, al Waer and Douma by the end of the week if the permits were issued, which would be easy to do.
"Can well-fed grown men please stop putting political, bureaucratic and procedural road blocks for brave humanitarian workers that are willing to go to serve women, children, wounded civilians in besieged and crossfire areas?" Mr Egeland said.