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.
And he defended the bombardment of Aleppo by Russian and Syrian forces.
Jabhat Fateh al-Sham is linked to al-Qaeda.
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."
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.
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 United Nations 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.
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.
"We are not using any munition which is prohibited by the United Nations," he added.