After Istanbul, Kerry will travel to Brussels, where he will discuss the Syria crisis with NATO and EU foreign ministers. He will also meet with Russian Foreign Minister Sergei Lavrov. The Obama administration sees Moscow, one of Syria's most important backers, as key to a political settlement.
On Thursday, Kerry said Washington was still open to negotiations between the regime and the opposition but warned "that time is not on the side of a political solution. It's on the side of more violence, more extremism, an enclave breakup of Syria."
The longer the war drags on, Kerry said, the greater the chance of a "very dangerous sectarian confrontation over the long term, and the potential of really bad people getting hold of chemical weapons," he added.