U.S. Secretary of State John Kerry says "substantial progress" has been made toward ensuring that Iran's nuclear program will not lead to weapons development.

But he says important gaps still stand in the way of an agreement.

Speaking after a week of nuclear talks with Iran, Kerry said Saturday that the U.S. wasn't rushing for an agreement even as a March 31 deadline looms for a framework agreement.

He said fundamental decisions needed to be made and stressed that world powers and Iran have an opportunity right now for a diplomatic solution.

Kerry was departing later Saturday for London to meet with European allies, before returning to Washington.

Talks with Iran resume next week.