Secretary of State John Kerry says the U.S., Iran, Russia and Europe agree Syria should be united and that Syrians should choose their own future leadership, but he says President Bashar Assad is the one obstacle to doing that.

Kerry is to meet Friday with counterparts from Turkey and Saudi Arabia, and from Russia, one of Assad's main backers.

Kerry said Thursday the various powers with an interest in resolving Syria's conflict agree on "unified principles." He added: "One thing stands in the way of being able to rapidly move to implement that and it's a person called Assad, Bashar Assad."

Kerry spoke alongside German Foreign Minister Frank-Walter Steinmeier, who said finding a road to a political solution "depends on whether Washington and Moscow find bridges to each other."