Secretary of State John Kerry is in Switzerland today for more talks with Iran's foreign minister on the Islamic republic's nuclear program. The meeting comes a week after Senate Republicans signed an open letter to Iran suggesting any deal reached would lapse the day President Obama leaves office.

NPR's Peter Kenyon, who is reporting on the talks for our Newscast unit, says:

"Kerry and [Iranian Foreign Minister Mohammed Javad] Zarif both say important gaps remain with two weeks remaining to reach a framework agreement on the major issues. These include Iran's ability to research and develop more advanced centrifuges and other nuclear technology, how long any agreement would last and how quickly sanctions will be lifted, providing some relief to Iran's struggling economy."

Last week's letter by 47 GOP senators is also likely to be discussed. Kerry said Saturday that he would seek to assure U.S. allies — Britain, China, France, Germany and Russia — and Zarif that Congress did not have the authority to change a deal — if one is reached. And, he told CBS News, in an interview aired today, that the letter was "wrong," "unprecedented" and "unconstitutional."

He added most of the differences the U.S. and its allies have with Iran are "political decisions that need to be made in order to fulfill the promise of proving to the world that a program is peaceful."

The letter's primary author, freshman Sen. Tom Cotton, R-Ark., defended his actions on CBS.

"The fact that President Obama doesn't see this letter as a way to get more leverage at the negotiating table just underscores that he is not negotiating for the hardest deal possible," he said on Face the Nation.

He was backed by Senate Majority Leader Mitch McConnell, who called the reaction to the letter selective outrage.

"The main point here I think everybody needs to understand is that the president is about to make what we believe will be a very bad deal," he said on CNN's State of the Union. "He clearly doesn't want Congress involved in it at all, and we're worried about it."

The White House wrote to the Senate Foreign Relations Committee on Saturday to say legislation being considered by the Senate would "potentially prevent any deal from succeeding."

"The administration's request to Congress is simple: Let us complete the negotiations before the Congress acts on legislation," White House chief of staff Denis McDonough said in the letter.

That committee's chairman, Sen. Bob Corker, R-Tenn., who was not one of the signatories to the GOP letter, said he believes "it is very important that Congress appropriately weigh in before any final agreement is implemented."

Copyright 2016 NPR. To see more, visit