Secretary of State John F. Kerry, left, and his Iranian counterpart, Mohammad Javad Zarif, resume nuclear talks in Switzerland before a March 31 deadline for a political agreement. (Reuters)

Iranian negotiators meeting with U.S. officials Monday expressed concern over a letter from Senate Republicans warning that a nuclear deal with President Obama might not outlast his time in office.

A senior administration official said the Iranians broached the subject in the almost five hours of discussions led by Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif. The letter also was raised in meetings Sunday between political and technical experts, the official said.

Iranian and U.S. officials have been critical of the open letter addressed to Iran’s leaders. It was written by Sen. Tom Cotton (R-Ark.) and signed by 46 other Republican senators. Kerry has said its claims that Congress can modify an executive agreement are inaccurate. He also has said that its predictions of a short shelf life for a nuclear deal with Iran undermine his diplomatic efforts.

“I’m sure you’ll be shocked to know the Cotton letter came up in the talks,” the senior administration official said, speaking on the condition of anonymity because of rules for briefing reporters.

The official would not characterize the tenor of the Iranian questions, other than to say they reflected “a concern.” But the official was dismissive of the letter’s overall effect on the talks, labeling it a “distraction, “unhelpful,” and “ill informed and ill advised.”

Freshman Sen. Tom Cotton (R-Ark.) told CBS News he stands behind his letter to Iran, saying it is critical to stopping that country from developing a nuclear weapon. (Reuters)

“These negotiations are not about the letter,” the official said. “The negotiations are about whether Iran can make the necessary decisions to assure the world its program is exclusively peaceful.”

[Read: The misguided, condescending letter from Republican senators to Iran]

The United States and five other world powers have been negotiating with Iran for almost a year and a half, aiming for a deal that would restrict its nuclear program. In return, the United States, the European Union and the United Nations would ease sanctions imposed on Iran. Iran says its program is intended for nonmilitary uses, but some countries worry that Tehran wants to eventually build nuclear weapons.

The negotiators face a self-imposed deadline of March 31, set when an interim agreement was extended in November. The United States wants a signed agreement on the outlines of a final deal, to be completed in late June, but Iran has said it wants only one final agreement put on paper.

[Read: Crunch time for talks]

“We are trying to get there. But, quite frankly, we still do not know if we will be able to,” the administration official said. “Iran still has to make some very tough and necessary choices to address the significant concerns that remain about its nuclear program.”

Comparing the negotiations to lining up the pieces of a Rubik’s Cube, the official added, “You can get very far, and the last pieces are just too hard.”

The official declined to say whether there was more reason for optimism than pessimism but described the pace of the talks as veering wildly, from hour to hour on some days.

“It’s a bit of a roller coaster,” the official said. “One day we feel we may get there. The next day, or hour, we might feel maybe not so much. That’s why it is very difficult and challenging.”

Zarif was in a more upbeat mood, shouting out to reporters, “Finally, finally, we’ll get there,” as he departed for Brussels on Monday afternoon to talk with E.U. officials and foreign ministers from the countries negotiating alongside the United States: Britain, France, Russia, China and Germany.

He is expected to return Monday night so talks can resume Tuesday in a hotel where the Treaty of Lausanne was signed in 1923, officially dividing the Ottoman Empire.

A sense of urgency permeated the meeting in Brussels.

Federica Mogherini, the E.U. foreign policy chief, said, “We are entering a crucial time, a crucial two weeks.”

German Foreign Minister Frank-Walter Steinmeier said, “We should seize this opportunity.”

Meanwhile, British Foreign Secretary Philip Hammond said significant differences remain.

“We are closer than we were, but we’ve still got a long way to go,” he told reporters. “There are areas where we’ve made progress, areas where we have yet to make any progress.”

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