<text>  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 Barack Obama leaves office, a senior U.S. official said Monday. The official, noting the administrations 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 an agreement to reduce Irans ability to produce a nuclear weapon in exchange for loosing Western sanctions. 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 in Lausanne, Switzerland led by Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif. Scroll down for video . U.S. Secretary of State John Kerry, left, listens to Irans Foreign Minister Mohammad Javad Zarif, right, before resuming talks over Irans nuclear program in Lausanne, Switzerland on Monday . 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 ten years of negotiations, we should seize this opportunity. The United States and Iran are plunging back into negotiations in a bid to end a decades-long standoff that has raised the specter of an Iranian nuclear arsenal, a new atomic arms race in the Middle East and even a U.S. or Israeli military intervention . U.S. Secretary of State John Kerry takes a break during a bilateral meeting with Iranian Foreign Minister Mohammad Javad Zarif over Irans nuclear program . There are areas where weve made progress, areas where we have yet to make any progress, British Foreign Secretary Philip Hammond said. But the fact that were all here talking shows the commitment on both sides to try to reach an agreement. In Lausanne, the U.S. official wouldnt 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 didnt 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. In the end, the talks and a potential agreement depend on Iran showing the world that its nuclear program is exclusively peaceful, said the U.S. official, who briefed reporters only condition of anonymity. The goal for a full agreement is the end of June. U.S. Energy Secretary Ernest Moniz, U.S. Secretary of State John Kerry, Irans Foreign Minister Mohammad Javad Zarif and the head of the Atomic Energy Organization of Iran Ali Akbar Salehi, from left to right, pose for a photograph before resuming talks over Irans nuclear program . Republicans argue that a deal would be insufficient and unenforceable, allowing Iran to eventually become a nuclear-armed state. To that end, theyve 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. Cottons letter, the administration and congressional Democrats argue, went further, interfering in the presidents 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 Obamas successor. Obama and other officials insist theyre not going to make any deal that would allow Iran to acquire nuclear weapons. The agreement taking shape would limit Irans uranium enrichment and other nuclear activity for at least a decade, with the restrictions slowly lifted over several years. From left, German Foreign Minister Frank-Walter Steinmeier, European Union High Representative Federica Mogherini, Irans Foreign Minister Mohammad Javad Zarif and British Foreign Minister Philip Hammond stand for a group photo prior to a meeting in Brussels on Monday . 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 weeks end for talks with Afghanistans 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 Americas 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 theyve made enough progress to justify further talks. Such a declaration would hardly satisfy U.S. critics of the Obama administrations 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. 