Earlier, Washington asked Moscow to direct its airstrikes solely at al-Nusra Front and refrain from targeting so-called "moderate" rebels, working with al-Qaeda's branch in Syria.
"The Americans are now saying that they are unable to remove the 'good' opposition members from the positions held by al-Nusra Front, and that they will need another two-three months. I am under the impression that there is a game here and they may want to keep al-Nusra Front in some form and later use it to overthrow the [Assad] regime," Lavrov said at the St. Petersburg International Economic Forum.
The US-Russian brokered ceasefire entered into force on ceasefire agreement came into force across Syria on February 27. Terrorist organizations such the Daesh and al-Nusra Front are excluded from the truce.