In a striking shift, President Barack Obama and Russian President Vladimir Putin are embarking on a tentative path toward closer ties and possible military cooperation, as the bitter rift over Ukraine gives way to common cause against the Islamic State group.

After months of accusing Moscow of propping up Syrian President Bashar Assad, Obama is changing his tune. He's praising Putin as a "constructive partner" in the diplomatic effort to resolve Syria's civil war.

Putin, too, is issuing conciliatory signals. He's calling for the U.S. and Russia to "stand together" against the extremist threat.

The prospect of U.S.-Russian military cooperation has seemed remote ever since the U.S. cut off military ties over Russia's actions in Ukraine. But Obama says it's possible, depending on Russia's stance on Syria's political transition.