"It does appear that they (Russians) were in areas where there probably were not IS forces, and that is precisely one of the problems with this whole approach," Defence Secretary, Ashton Carter said at a Pentagon news briefing.
Carter said Russia states an intent to fight IS on the one hand, and to support embattled Syrian President Bashar al-Assad and his regime, on the other.
"Fighting IS without pursuing a parallel political transition only risks escalating the civil war in Syria -- and with it, the very extremism and instability that Moscow claims to be concerned about and aspire to fighting," he said.
"We will continue to insist on the importance of simultaneously pursuing these two objectives. And I would hope that Russia would join us in pursuing these objectives, which they claim to share in parallel, rather than in a sequence that cannot succeed," Carter said.
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Secretary of State John Kerry echoed Carter at the UN in New York. The US supports any genuine effort to fight IS and al-Qaeda-affiliated groups, especially al-Nusrah, he said.
"But we must not and will not be confused in our fight against IS with support for Assad. Moreover, we have also made clear that we would have grave concerns should Russia strike areas where IS and al-Qaeda-affiliated targets are not operating. Strikes of that kind would question Russia's real intentions in fighting IS or protecting the Assad regime," the top American diplomat said.
"They've been supporting him for quite some time, and it's clear that they've made a significant military investment now in further propping him up," he said.
"The fact that Russia has to take these noteworthy steps to ramp up their support for Assad is an indication of how concerned they are about losing influence in the one client state that they have in the Middle East," he added.
This, he said is in contrast to what President Barack Obama discussed with his Russian counterpart Vladimir Putin.
"That means Russia won't succeed in imposing a military solution on Syria any more than US was successful in imposing a military solution on Iraq a decade ago, and certainly no more than Russia was able to impose a military solution on Afghanistan three decades ago," Earnest said.