By Paul Richter, Tribune Washington Bureau (TNS)

WASHINGTON — A defiant letter from Senate Republicans to Iran’s leaders has added a new complication to the fraught international negotiations seeking to end the decade-long standoff over Iran’s nuclear program.

As U.S. and Iranian officials began a weeklong push to complete at least the outline of a deal, Tehran repeatedly demanded an explanation of the letter written last week by Senator Tom Cotton (R-AR) and signed by 47 of the 54 Republican senators, a senior Obama administration official said.

The open letter warns Iranian leaders that Congress can block a nuclear pact that the administration negotiates if lawmakers consider it too lenient, and that an agreement could effectively expire once President Barack Obama leaves office.

The administration official said Iranian officials raised the letter in a negotiating session on Sunday, and again on Monday, as the two sides sat through five hours of talks in a luxury hotel on the edge of Lake Geneva.

“These kinds of distractions are not helpful when we’re talking about something so serious,” said the senior administration official, who declined to be identified under ground rules set by the administration. “It was of concern. … It is an issue.”

The official declined to describe how U.S. officials responded or to provide other details of the discussions. The official said the contentious issue didn’t stop the two sides from trying to resolve other differences still on the table.

Senior diplomats and technical experts met on Sunday. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif were scheduled to meet Monday.

The GOP letter has further inflamed an already passionate debate in Washington over leaked details of the expected framework agreement, which negotiators hope to finish this week if possible.

White House officials have sharply criticized the lawmakers’ letter as an unprecedented interference in the president’s prerogative to negotiate agreements and conduct foreign policy.

But others, including Cotton, insist that the warning was justified if it helps prevent a deal that could open the way for Iran to someday build a nuclear weapon.

Appearing on CNN on Sunday, Senate Majority Leader Mitch McConnell defended the letter, accusing Democrats of selective outrage and warning of a “very bad” nuclear deal. Cotton said he had no regrets, saying the fierce criticism showed that Obama wasn’t negotiating “the hardest deal possible.”

The United States and five other world powers — France, Britain, Germany, Russia and China — have been negotiating as a group for about 18 months to try to curb Iran’s nuclear program.

Officials have said the deal under consideration would see Iran freeze its nuclear program for at least a decade, with restrictions then lifted over an additional five years or so. In exchange, the United States 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 only wants to enrich uranium to generate electricity and for other peaceful purposes.

The two sides hope to complete an outline of a deal by the end of this month, and then work out details of a comprehensive step-by-step agreement by June 30.

Officials on both sides would like to complete the framework this week, however, so they can adjourn before the Iranian new year celebration, which begins Saturday. But the senior administration official made no promises.

Photo: U.S. Mission/Eric Bridiers via Flickr