The US is trying to spare a Syrian jihadist group in case it is needed to unseat President Assad, the Russian 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 US-backed moderate rebels.
"They still are not able or willing to do this," he said.
"We believe the plan was to spare Nusra."
He added: "They [have] never touched Nusra anywhere in Syria."
Mr Lavrov was speaking on the first anniversary of the beginning of the Russian air campaign in Syria.
He defended the Russian bombardment of the besieged city of Aleppo by Russian and Syrian government forces.
The United Nations says 400 civilians, including many children, have been killed in the city during the past week.
Mr Lavrov insisted Russia was helping President Bashar al-Assad's forces to "fight terrorists".
And he accused the West of staying quiet about civilian suffering in Aleppo when it expected the city to fall to the rebels.