U.S. Secretary of State John Kerry said he's returning to nuclear negotiations with Iran with "important gaps" standing in the way of a deal, the Associated Press reports.

Kerry said he can't assure whether the U.S. and other world powers will be able to reach a framework deal with Iran before a March 31 deadline, adding that he needs to speak with Iran's foreign minister to assess the effects of a letter from Republicans in the U.S. Congress to Iran's leaders.

Emphasizing that the letter undermined the Obama administration's diplomatic effort, Kerry also repeated the administration's stand that it wants the right deal with Iran, not any deal.

He spoke Saturday, March 14, in the Egyptian resort of Sharm el-Sheikh, where he attended an economic conference. He heads to Switzerland on Sunday for the nuclear talks.