Secretary of State John Kerry, returning to talks with Iran on its nuclear program, said Sunday that most of the differences still barring an agreement are political rather than technical.

Kerry, who was to sit down later Sunday in Switzerland with Iranian Foreign Minister Mohammed Javad Zarif, said "there are clearly some differences that still rest on a technical judgment."

"But by and large, most of the differences now are political decisions that need to be made in order to fulfill the promise of proving to the world that a program is peaceful," he added. Kerry said in an interview on CBS News that Tehran "to its credit has thus far lived up to every part of the agreement we made over a year ago."

His interview was aired a day after White House chief of staff Denis McDonough 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 -- but not Corker -- sent a letter to Iranian officials earlier asserting that Congress must have a say in approving any agreement. Corker has expressed separate concerns about Congress being denied a part in the process.

McDonough told Corker that legislation sponsored by the Tennessee Republican would go far beyond ensuring a role for Congress in any deal with Iran.

"Instead, the legislation would potentially prevent any deal from succeeding by suggesting that Congress must vote to `approve' any deal," McDonough said. He criticized a provision that would eliminate President Barack Obama's authority to lift some sanctions on Iran as part of any agreement.

In the CBS interview, Kerry declined to comment on speculation about the nature of any nuclear program that Iran would have at the end of 10 or 15 years under a negotiated agreement.

"I'm not going to get into the end of the deal or the beginning of the deal, or how long it is, or what the framework is. That is what we are negotiating," the secretary said.

"The proof will be in the pudding, but nobody should be jumping to a conclusion as to what the breadth and framework of this agreement is because it is not yet finalized."

Kerry said he doesn't know yet whether the letter sent to Iranian officials by Senate Republicans has jeopardized the talks, but said he has no doubt that it was calculated to interfere with negotiations.

If an agreement is not reached by the deadline set for the end of March, Kerry suggested that an extension was unlikely. He noted that discussions about developing a framework for proving that the Iranian nuclear program is peaceful have been going on for two years.

"We believe very much that there is not anything that is going to change in April or May or June, that suggests that at that time the decision you can't make now will be made then," he said. "If it's peaceful let's get it done. My hope is that in the next days that will be possible."

In his letter to Corker, McDonough said, "The administration's request to Congress is simple: Let us complete the negotiations before the Congress acts on legislation." He added that he does expect a robust congressional debate if a final deal is struck by the end of June.

And McDonough reiterated Obama's repeated threats to veto the legislation should Congress pass it.

Corker and Senate colleagues in both parties insist that Congress be allowed to consider and vote on any agreement designed to block Iran from developing a nuclear weapon. Corker argued his case in a letter to Obama last week, and did so again in response to McDonough.

"On this issue where Congress has played such a vital role, I believe it is very important that Congress appropriately weigh in before any final agreement is implemented," Corker said in a statement late Saturday.

Tensions between the administration and lawmakers over Iran have been rising for weeks.