GENEVA (AP) -- Nuclear talks with Iran have failed to reach agreement, but U.S. Secretary of State John Kerry says differences between Tehran and six world powers made "significant progress."

Kerry spoke early Sunday to reporters after more than three days of efforts to seal a deal meant to start limiting Iranian nuclear activities in exchange for some sanctions relief on Tehran.

Kerry and counterparts from the five other nations joined negotiators starting Friday, but even their authority failed to resolve the differences.

Despite the lack of agreement, Kerry says the negotiators "not only narrowed the differences ... but we made significant progress."

Kerry touched on reports of differences with France, saying the U.S. shared some French concerns. He said, "We are grateful to the French for the work we did together."