WASHINGTON - With another round of nuclear talks set to begin this week between the United States and Iran, the top American diplomat said the obstacles to an agreement are largely political, rather than technical.

“There are clearly some differences that still rest on a technical judgment,” Secretary of State John Kerry said in Sunday on CBS television’s Face the Nation news programme. “But by and large, most of the differences now are political decisions ... to fulfil the promise of proving to the world that a program is peaceful.”

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.

Kerry will meet in Geneva with Iranian counterpart Mohammed Javad Zarif in the next round of talks on the future of Iran’s nuclear programme.

Meanwhile, US Senate Republican leader Mitch McConnell issued a statement against the talks, saying President Barack Obama was on the cusp of a “very bad deal” with Tehran.

Kerry said he is unsure if an open letter sent by McConnell and 46 other Republican US senators last week to Iranian leaders had jeopardized the future of the ongoing talks.

The group warned that the next US president could revoke a deal at any time, a statement the secretary of state challenged. The United States and its partners in the P5+1 group, Britain, China, France, Russia and Germany, have an end-of-March deadline to reach a framework deal for Iran to cut back its uranium enrichment programme.

Tehran said economic sanctions must be lifted for a deal to be achieved; the country insists its nuclear programme is strictly for civilian purposes.

US lawmakers who oppose the deal claim Iran cannot be trusted.

The White House has confirmed that any nuclear agreement between Iran and the P5+1 group of world powers would be subject to a vote by the UN Security Council. The acknowledgement by President Barack Obama’s chief of staff Denis McDonough comes as the White House butts heads with Republicans over whether the US Congress should vote on any deal.

“Just as it is true that only Congress can terminate US statutory sanctions on Iran, only the Security Council can terminate the Security Council’s sanctions on Iran,” McDonough said in a letter on Saturday to Bob Corker, the Republican head of the Senate Foreign Relations Committee.

In the CBS interview, Kerry declined to comment on speculation about the nature of any nuclear programme 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.