Russian Foreign Minister Sergei Lavrov has accused the United States of trying to protect a militant extremist group in Syria in its effort to overthrow the regime of Syrian President Bashar al-Assad.
Lavrov made the accusations in an interview with BBC World News on September 30.
He said Washington had vowed to "take as a priority an obligation to separate" some opposition fighters in Syria from the former Al-Qaeda affiliate once known as Al-Nusra Front.
But Lavrov accused the United States of failing to do so.
The Russian foreign minister said Russia has "more and more reasons to believe that from the very beginning the plan was to spare Al-Nusra and to keep it just in case for Plan B or stage two, when it would be time to change the regime."
Moscow has repeatedly accused the United States of failing in its commitment to persuade rebel forces to distance themselves from militant groups such as Al-Nusra, which changed its name to Fateh al-Sham Front in July after renouncing its ties to Al-Qaeda.
Fighters from the Al-Nusra Front in the northern Syrian city of Aleppo in May 2015
Lavrov told the BBC he would speak with Kerry about the issue again on September 30.
A spokesman for the U.S. State Department said on the same day that it was difficult to continue to believe in a diplomatic process for Syria considering the events happening there on the ground.
Meanwhile, Russian news agencies on September 30 quoted Lavrov as saying that a cease-fire deal on Syria that Moscow agreed with Washington was still working.
He also said Russia was not using banned weapons in Syria and demanded evidence from those accusing Moscow of hitting civilian targets with air strikes in Aleppo's opposition-controlled east.
U.S. Secretary of State John Kerry said on September 29 that Washington was on the verge of suspending talks with Russia cooperation in Syria over the collapse of the cease-fire on September 19.
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Since then, Russian and Syrian warplanes have unleashed devastating bombardments of rebel-held neighborhoods in the city of Aleppo.
Together with rebel fighters, some 250,000 civilians are surrounded by Syrian government forces in those parts of Aleppo.
Kerry's comments came hours after the Kremlin dismissed an earlier U.S. threat to suspend cooperation with Moscow in Syria, and said Russian forces would continue to support a government offensive on rebel-held neighborhoods of Aleppo.
With reporting by Reuters, AP, AFP, BBC, Interfax, and TASS