Top U.S. and Iranian diplomats returned to talks Tuesday, seeking to resolve differences blocking a deal that would curtail Iran's nuclear program and ease sanctions on the country. Among the issues they're now contending with is a Republican letter warning that any deal could collapse the day President Barack Obama leaves office.

The discussions between Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif came after a senior U.S. official described Iranian diplomats twice confronting their American counterparts about last week's open letter to Iran's leaders written by freshman Sen. Tom Cotton of Arkansas and signed by 46 other GOP senators.

The letter came up in talks Sunday between senior U.S. and Iranian negotiators, the official said, and the Iranians raised it again in negotiations Monday led by Kerry and Zarif.

The official described the Republican intervention as a new challenge for negotiators facing an end-of-month deadline for a framework accord. Zarif confirmed that it was on his mind, telling Iranian state media: "It is necessary that the stance of the U.S. administration be defined about this move." The U.S. official wasn't authorized to speak publicly on the matter and briefed reporters on condition of anonymity.

Kerry and Zarif met for nearly five hours in the Swiss city of Lausanne Monday, before the Iranians departed for Brussels for talks with European negotiators.

There, EU foreign policy chief Federica Mogherini said "we are entering a crucial time, a crucial two weeks." German Foreign Minister Frank-Walter Steinmeier said after "more than 10 years of negotiations, we should seize this opportunity." British Foreign Secretary Philip Hammond said all sides were committed to trying. China, France and Russia are also involved in the negotiations.

As U.S.-Iranian talks resumed Tuesday in Lausanne, the Republican letter threatened to cast a shadow on the negotiation. The senior American official wouldn't say how much time the sides spent talking about it in the last two days, but since the letter's release seven days ago, the Iranians have called it a propaganda ploy and highlighted the internal U.S. division.

Republicans argue 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. And the senior U.S. official emphasized 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 accord is the end of June.

The deal 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.