In an interview aired Friday on BBC World News, Lavrov said Washington had vowed to "take as a priority an obligation to separate the opposition" from the former Al-Qaeda affiliate once known as Al-Nusra Front, but that it had not done so.
"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," Lavrov said.
A short-lived truce brokered by Moscow and Washington earlier this month could have led the two countries to coordinate strikes against jihadists, but the deal fell apart as both sides blamed each other for its failure.
Moscow has been accused of indiscriminately bombing Aleppo's opposition-controlled east as it helps an assault currently being conducted by Syrian government troops to capture all of the country's second city.
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Russia has said that it would continue its Syrian bombing campaign in spite of US warnings that Washington would suspend talks unless Moscow stopped its assault on Aleppo.
The British-based Syrian Observatory for Human Rights said Friday that more than 9,300 people -- including some 3,800 civilians -- had been killed in the year of Russian air strikes in Syria in support of Assad.
Moscow quickly dismissed the claim, insisting its bombing campaign had prevented jihadists from taking over in the war-torn country.