Iranian officials reportedly have confronted their U.S. counterparts twice over an open letter from Republican senators to Tehran that warned any agreement on Iran's nuclear program would be unlikely to last beyond President Barack Obama's term of office. 

The Associated Press, citing a senior U.S. official, reported that the letter, which was signed by 47 of  54 GOP members, first came up in negotiations on Sunday and was raised again Monday in discussions led by Secretary of State John Kerry and Iranian Foreign Minister 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.

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 that "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, R-Ark. 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 wasn't authorized to speak publicly on the matter and briefed reporters only on 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, R-Tenn., 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.

The Associated Press contributed to this report.