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, U.S. officials said Monday.

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

The letter came up in nuclear talks Sunday between senior U.S. and Iranian negotiators, the officials 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.

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, a senior 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 Republican 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.

In Washington, State Department spokeswoman Jen Psaki said, “Negotiations in our view, and I think most people’s view, are not about a letter that was ill-informed and ill-advised.” “We certainly anticipate that the focus of the discussions will remain on the issues at hand,”she told reporters.

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 in Lausanne only condition of anonymity.