The U.S. 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, the news agency reports.

Sergei Lavrov said the U.S. had broken its promise to separate the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) and other groups such as Islamic State from U.S.-backed moderate rebel groups.

"They still are not able or willing to do this... we believe the plan was to spare Nusra," he said.

He was speaking on the first anniversary of the beginning of the Russian air campaign in Syria.