The United States would have “grave concerns” if Russia used its air strikes in Syria to support President Bashar Assad against groups other than Islamic State or al Qaida, secretary of state John Kerry has said.
He spoke as Russia launched its first air strikes in Syria targeting what it said were Islamic State positions.
US officials and others cast doubt on that claim, saying the Russians appeared to be attacking opposition groups fighting Syrian government forces.
On Wednesday, some US-backed rebel groups claimed they were hit by Russian air strikes but those claims could not be confirmed.
A moderate Western-backed rebel group said one of its leading officers was killed in central Homs province.
At the White House, presidential spokesman Josh Earnest also expressed concern over Russia’s actions.
“We are seeing the Russians ramp up their support for President Assad,” Mr Earnest said.
“They’ve been supporting him for quite some time. It’s clear they’ve made a significant military investment now in further propping him up.”
Mr Earnest said the new action “calls into question their strategy, because when President Putin and President Obama had the opportunity to meet at the UN earlier this week much of their discussion was focused on the need for a political transition inside Syria.”
Mr Kerry said Russian operations must not support Mr Assad or interfere with those of the US-led coalition that is already attacking Islamic State targets.
He called for an urgent start to military-to-military talks to prevent any kind of conflict between Russia and the coalition, suggesting they begin this week.
“If Russia’s recent actions and those now ongoing reflect a genuine commitment to defeat (the Islamic State) then we are prepared to welcome those efforts and to find a way to de-conflict our operations and thereby multiply military pressure on Isil and affiliated groups,” Mr Kerry said.
“But we must not and will not be confused in our fight against Isis with support for Assad.”
Mr Kerry also said the US-led coalition would “dramatically accelerate” its efforts.
Russian Foreign Minister Sergey Lavrov followed Mr Kerry, saying Russia is ready to “forge standing channels of communication to ensure a maximally effective fight.”
He listed countries with a key role to play in resolving the chaos in Syria, including Iran, Russia, Saudi Arabia, Turkey, Egypt, Jordan, Qatar , the US and China.