Russia's foreign minister says he believes the U.S. may hope to use al-Qaida's branch in Syria to unseat Syrian President Bashar Assad's government.

Speaking Thursday at an economic forum in St.Petersburg, Sergey Lavrov said the reluctance of U.S.-backed Syrian opposition groups to distance themselves from al-Qaida's branch in Syria, the Nusra Front, has been a major reason behind continuing fighting.

He said the U.S. could be "playing some kind of game here, and they may want to keep Nusra in some form and use it to topple the regime."

Lavrov was commenting on U.S. Secretary of State John Kerry's warning that Washington is losing its patience with Russia, as the U.S.- and Russia-brokered cease-fire in Syria was at risk mainly due to violations by Assad's forces with Russian air support.