The Obama administration is warning Congress once more to stand down from interference with U.S. negotiations with Iran over its nuclear program.

White House chief of staff Denis McDonough sent a letter to Senate Foreign Relations Committee Chairman Bob Corker, asserting that pending legislation in Congress likely would have a "profoundly negative impact" on the ongoing talks the U.S. government is having with Tehran.

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

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"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.

The talks are to resume Sunday in Switzerland, with the U.S. and other world powers facing an end-of-March deadline to reach a framework deal.

Secretary of State John Kerry said Saturday he doesn't know yet whether a letter sent to Iranian officials by more than 40 Senate Republicans has jeopardized talks over Iran's nuclear program but that he has no doubt it was calculated to interfere with negotiations.

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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," Kerry told CBS' "Face the Nation" in an interview airing Sunday. "If it's peaceful let's get it done. My hope is that in the next days that will be possible."

Kerry was in Egypt on Sunday for an economic conference before preparing for another round of discussions later that day in Switzerland with Iranian Foreign Minister Mohammad Javad Zarif.

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.

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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.