Russian President Vladimir Putin has said Syrian rebel groups are exploiting the current ceasefire to regroup.
He said the US, which backs some rebel groups, was more focused on retaining their military capacity than separating moderate and militant groups, a stated aim of the truce which began on Monday.
Mr Putin also urged Washington to make the ceasefire agreement public.
There are growing tensions between the two major powers over the rebels’ role and delays in providing aid.
Separately footage has emerged of Free Syrian Army rebels expelling US special forces from the town of Al-Rai in northern Syria, calling them “infidels” in Arabic.
The Pentagon said on Friday that dozens of US troops had been deployed to the border with Turkey at Ankara’s request, to fight so-called Islamic State (IS) militants.
In televised remarks during a visit to Kyrgyzstan, Mr Putin said Moscow was keeping to its obligations under the Syrian truce, and the Syrian government was “fully abiding” by the agreement.
But he said that the US seemed to be trying to keep the rebels’ military capability in its conflict with the government, adding that this was a “dangerous path”.
“What we’re seeing now is not the separation of the terrorists from the healthy part of the opposition but an attempt by these terrorists to regroup,” he said.
Previously Moscow has threatened to resume air strikes on “moderate” rebel groups if this separation did not happen.
Mr Putin added that he could not understand why the US had not shared details of the ceasefire agreement with the UN Security Council.
A meeting of the Council scheduled for Friday was cancelled at the last minute as a result.
The Russian president said Moscow would disclose nothing without US consent, adding that he was “more positive than negative” about a UN resolution being agreed.
The US has voiced concern about delays in providing humanitarian aid to Aleppo and other besieged areas.
Breaches of the truce by Syrian troops and rebel groups have also been reported.
However, both the US and Russia have said they want to extend the ceasefire.
If the cessation holds for seven days, the US and Russia have agreed to plan joint attacks on the Jabhat Fateh al-Sham group, which was previously known as the al-Nusra Front, and IS.
The UN says it is still waiting to be able to deliver aid to besieged Aleppo.
Some 20 trucks have been waiting for safe passage to cross from Turkey into Syria and on to rebel-held east Aleppo since Monday.
However, the UN says it has not yet received permits from the Syrian government to allow the trucks into opposition areas, where at least 250,000 people are in desperate need of food and medicine.