US President Barack Obama said Thursday that he was "not confident" that Russia and President Vladimir Putin could be trusted as partners in trying to end Syria's bloody civil war.

"We go into this without any blinders on. We're very clear that Russia has been willing to support a murderous regime, and an individual," Obama said, referring to Syrian President Bashar al-Assad.

"I'm not confident that we can trust the Russians and Vladimir Putin," he told reporters.