A defiant letter from Senate Republicans to Iran’s leaders is complicating international negotiations to end a 12-year international impasse by imposing strict limits on Iran’s nuclear program in exchange for easing economic sanctions.

As U.S. and Iranian officials began a weeklong push to complete the outline of an agreement, Tehran repeatedly demanded an explanation of the letter written by freshman Sen. Tom Cotton, R-Ark., and signed by 47 of the 54 Republican senators, a senior Obama administration official said.

The open letter, released last week, warned Iranian leaders that any accord reached by President Obama without congressional approval could be voided “with the stroke of a pen” by a future president, and that future Congresses could modify the agreement “at any time” if lawmakers consider it too lenient.

Iranian officials raised the letter in a negotiating session Sunday, and again Monday, according to the administration official. The two sides met behind closed doors for five hours in a luxury hotel here.

“These kinds of distractions are not helpful when we’re talking about something so serious,” said the official, who declined to be identified under administration ground rules.

The official declined to say how U.S. officials responded, or provide other details. The official said the contentious issue didn’t stop the two sides from trying to resolve other differences still under negotiation two weeks before a self-imposed deadline.

Appearing Sunday on CNN, Senate Majority Leader Mitch McConnell of Kentucky defended the letter and warned that the administration is negotiating a “very bad” nuclear deal. Cotton said he had no regrets about the letter, saying the fierce criticism it sparked showed that President Obama wasn’t negotiating “the hardest deal possible.”

Secretary of State John Kerry met with Iranian Foreign Minister Mohammad Javad Zarif here Monday after separate talks by senior diplomats and technical experts Sunday.

The five permanent members of the United Nations Security Council — the United States, France, Britain, Russia and China — plus Germany have been negotiating as a group for about 18 months to try to restrain Iran’s nuclear program.

The two sides initially had sought to complete a deal by last July, but that deadline was pushed back. Diplomats now aim to finish an outline this month and then work out the technical details and drafting of a final nuclear accord by June 30. Both sides have said they won’t seek further ­extensions.

Both sides would like to complete the framework this week, if possible, so they can adjourn before the Iranian New Year celebration, which begins Saturday.

Officials have said the deal under consideration would see Iran freeze most of its nuclear enrichment program for at least a decade, with restrictions then lifted over a further five years or so. In exchange, the U.S. and other countries would gradually lift economic sanctions but maintain monitoring of nuclear facilities.

Iran denies it plans to build a nuclear weapon, and says it wants to enrich uranium to generate electricity and for other peaceful purposes only.

Cotton’s letter has further inflamed an impassioned debate in Washington over the negotiations. Critics say concessions by the U.S. and the other world powers would not prevent Iran from building a nuclear weapon one day should it choose to do so.