The US is trying to spare a jihadist group in its attempts to unseat Syria's President Bashar al-Assad, Russia's foreign minister has told the BBC.
Sergei Lavrov said the US had broken its promise to separate the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) and other extremist groups from more moderate rebels.
Jabhat Fateh al-Sham is linked to al-Qaeda.
A US state department spokesman said the Russian allegations were "absurd".
Mark Toner told journalists that the US had not targeted al-Nusra for months because they had become "intermingled" with other groups and civilians.
And he accused the Russians of forcing moderate elements within the Syrian opposition into the hands of extremists with their attacks.
Mr Lavrov was speaking to Stephen Sackur on BBC World News TV on the first anniversary of the beginning of the Russian air campaign in Syria.
"They [the US] pledged solemnly to take as a priority an obligation to separate the opposition from Nusra," he said.
"They still, in spite of many repeated promises and commitments... are not able or not willing to do this and we have more and more reasons to believe that from the very beginning the plan was to spare Nusra and to keep it just in case for Plan B or stage two when it would be time to change the regime."
No credible 'plan B' - Diplomatic correspondent Jonathan Marcus
Mr Lavrov says that it is US policy towards Syria that is floundering, insisting that American officials have lost control of both events and of themselves.
There is an element of truth here - at least in policy terms. The US has no real alternative to Secretary of State John Kerry's efforts to deal with the Russians. There is no credible "plan B".
Mr Lavrov's central message - that Washington has refused to press its allies to separate themselves from the Islamists of al-Nusra ignores the fact that it is Russia's air campaign that is pushing rebel groups into al-Nusra's arms. Mr Lavrov's contention that the US is preserving al-Nusra, hoping eventually to use it to change the regime, will prompt gasps of condemnation in Washington.
But Mr Lavrov implicitly highlights a perennial difficulty for Washington - its search for a moderate opposition of sufficient critical mass to influence the battlefield.
A recent US-Russian deal was meant to lead to joint Russian-US air strikes on the Islamic State group and Jabhat Fateh al-Sham.
But many of the more moderate rebel groups that the US backs have formed a strategic alliance with the more powerful Jabhat Fateh al-Sham and now fight alongside it.
Mr Lavrov said: "We believe that the Russian-American deal must be put into effect. For this the only thing which is necessary is to separate the opposition from Nusra. If it is supported by the United States, not on paper but in real life, and then we will insist on an immediate cessation of hostilities."
The UN says 400 civilians, including many children, have been killed in the besieged city of Aleppo during the past week as a result of bombardment by Russian and Syrian government forces.
Mr Lavrov insisted Russia was helping President Assad's forces to "fight terrorists".
And he accused the West of staying quiet about civilian suffering in Aleppo when it was expecting the city to fall to the rebels after the Nusra group moved in and cut supply lines to the civilian population.
"We had many pauses, many humanitarian pauses during this year... 48 hours, 72 hours at the request of the United Nations.
"Every time these pauses have been used by Nusra to get from abroad more fighters, more ammunition and more weapons. There must be some first step and we have to get our priorities right.
"Humanitarian things are very important and we are doing everything now together with the Syrian government to help the United Nations to get weekly pauses in Aleppo to deliver humanitarian goods. It's the Nusra-controlled people in eastern Aleppo who refuse to do this."
Pressed on the civilian casualties in Aleppo allegedly caused by Russian bunker-busting bombs, phosphorus munitions and cluster bombs, he replied: "If this happens, then we are very sorry."
But he insisted there had been no "meaningful proof" of this and there was a need to investigate every case.
US Secretary of State John Kerry warned on Thursday that the US was "on the verge" of suspending talks with Russia over Syria.
Mr Toner told reporters in Washington that Mr Kerry had spoken to Mr Lavrov by telephone on Friday, but that Russia still had not taken steps to end violence in Aleppo.
"We are at the same place," he said.
Friday saw Syrian government forces and rebels fighting in the centre of Aleppo and north of the city, a week into the Russian-backed offensive by the Syrian army to take the city and surrounding area.
The Syrian Observatory for Human Rights monitoring group and a Syrian military official said government forces had captured territory north of Aleppo and buildings in the city centre, but rebel sources denied this.