US and Russia moving closer to proxy war in Syria

According to the New York Times, extreme militants, al-Nusra, were among the groups benefitting from increased weapons supplies to rebels; and that the FSA are in a tactical alliance with Al-Nusra .
According to Reuters, al-Nusra, using TOW missiles, were a driving force behind the major rebel advances in Idlib Province, near Assad’s heartland, which might have prompted Russia’s recent intervention.
Is it realistic to say that heavy weaponry supplied to moderates will not end up in hands of extremists? We are close to proxy war between America and Russia.