LAUSANNE, Switzerland - Iranian diplomats twice confronted their American counterparts about an open letter from Republican senators who warned that any nuclear deal could expire the day President Obama leaves office, a senior U.S. official said Monday.

The official, noting the administration's warnings when the letter first surfaced, said the GOP intervention was a new issue in the tense negotiations facing an end-of-month deadline for a framework agreement.

The Lausanne talks resumed Tuesday.

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The letter came up in nuclear talks Sunday between senior U.S. and Iranian negotiators, the official said, and the Iranians raised it again in discussions Monday led by Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif.

Zarif was quoted by Iranian state media after the meeting as saying the topics included the potential speed of a softening of U.S. economic sanctions and the new issue of the letter from the senators. "It is necessary that the stance of the U.S. administration be defined about this move," he was quoted as saying.

Over the weekend, Kerry told CBS News correspondent Margaret Brennan that he would not apologize for the letter, sent by 47 Republican lawmakers.

"Not on your life," Kerry said. "I'm not going to apologize for an unconstitutional, un-thought-out action by somebody who's been in the United States Senate for 60-some days. That's just inappropriate."

Kerry continued, "I will explain very clearly that Congress does not have the right to change an executive agreement."

Kerry and Zarif met for nearly five hours in Lausanne, the start of several planned days of discussions. Most of the Iranians then departed for Brussels, where they were to meet with European negotiators.

In Brussels, EU foreign policy chief Federica Mogherini said, "We are entering a crucial time, a crucial two weeks." And German Foreign Minister Frank-Walter Steinmeier said that after "more than 10 years of negotiations, we should seize this opportunity."

"There are areas where we've made progress, areas where we have yet to make any progress," British Foreign Secretary Philip Hammond said. "But the fact that we're all here talking shows the commitment on both sides to try to reach an agreement."

In Lausanne, the U.S. official wouldn't say how much time the sides spent talking about the letter drafted seven days ago by freshman Sen. Tom Cotton of Arkansas and signed by 46 other GOP senators. The Iranians have called the letter a propaganda ploy, and Zarif joked last week that some U.S. legislators didn't understand their own Constitution. The Obama administration has called the letter "ill timed" and "ill advised," coming weeks before the deadline for a preliminary agreement with Iran on its nuclear program.

Cotton isn't backing down. In his maiden speech in the Senate, Cotton reiterated his view that the deal being discussed would pave Iran's path to a nuclear bomb.

"Iran is an outlaw regime. ... Unsurprising, Iran is only growing bolder and more aggressive as America retreats from the Middle East," Cotton said, adding that Iranian leaders continue to call for Israel's elimination and that Iran is meddling in other nations, including Syria and Iraq.

The U.S. official in Lausanne, who briefed reporters only condition of anonymity, said that in the end, the talks and a potential agreement depend on Iran showing the world that its nuclear program is exclusively peaceful.

The goal for a full agreement is the end of June.

Republicans argue that a deal would be insufficient and unenforceable, allowing Iran to eventually become a nuclear-armed state. To that end, they've delivered a series of proposals to undercut or block an agreement, including ones that would require Senate say-so on a deal and order new sanctions against Iran while negotiations are underway.

Cotton's letter, the administration and congressional Democrats argue, went further, interfering in the president's execution of U.S. foreign policy. The letter, styled as a U.S. civics lesson, warned Iranian leaders that any deal negotiated by the current administration could be tossed by Obama's successor.

Obama and other officials insist they're not going to make any deal that would allow Iran to acquire nuclear weapons.

The agreement taking shape would limit Iran's uranium enrichment and other nuclear activity for at least a decade, with the restrictions slowly lifted over several years. Washington and other world powers also would gradually scale back sanctions that have crippled the Iranian economy. Tehran says it is only interested in peaceful energy generation and medical research, but much of the world suspects it harbors nuclear weapons ambitions.

Kerry and Zarif plan to regroup in Lausanne on Tuesday. The U.S. secretary of state is to return to Washington by week's end for talks with Afghanistan's leaders, and the Iranians plan to break for the Persian New Year. Officials say talks might restart sometime next week, if necessary. A deal would also require the approval of America's negotiating partners: Britain, China, France, Germany and Russia.

With little time remaining before the end of March, some officials have said the persistent differences mean negotiators will likely settle for an announcement that they've made enough progress to justify further talks.

Such a declaration would hardly satisfy U.S. critics of the Obama administration's efforts. But the senior American official said the goal was to determine by the end of March "if we can get to a political framework that addresses the major elements of a comprehensive deal."

Back in Washington, Sen. Bob Corker, chairman of the Senate Foreign Relations Committee, said his panel will likely vote on a bill next week that would require a congressional review of any nuclear deal. The measure would require the president to submit the text of any pact to Congress and bar the administration from suspending congressional sanctions on Iran for 60 days. In that time, Congress would hold hearings and have a chance to approve, disapprove or take no action on the agreement.