Secretary of State John Kerry, returning to talks with Iran on its nuclear program, says most of the differences still barring an agreement are political rather than technical.
Kerry, who is to sit down Sunday in Switzerland with Iranian Foreign Minister Mohammed Javad Zarif, tells CBS News that Tehran must fulfill "the promise of proving to the world" that its nuclear program is peaceful.
The secretary's interview is being aired a day after the Obama White House sent a letter to Senate Foreign Relations Committee Chairman Bob Corker warning Congress once more that it should not interfere in the negotiating process. More than 40 Senate Republicans sent a letter to Iranian officials earlier this month asserting that Congress must have a say in approving any agreement.