Talks on Iran's nuclear program resumed on March 26 as a deadline for a framework agreement drew nearer.
U.S. Secretary of State John Kerry and Energy Secretary Ernest Moniz met their Iranian counterparts, Foreign Minister Mohammad Javad Zarif and Atomic Energy Organization chief Ali Akbar Salehi, in the Swiss city of Lausanne.
Six world powers and Iran are facing a self-imposed March 31 deadline to reach a framework deal to limit Tehran's nuclear activities in exchange for relief from economic sanctions, with a full deal to follow by June 30.
Tehran denies it is secretly developing a nuclear weapon, as Israel and some Western nations suspect.
The talks began as the United States and Iran set out opposing stances in Saudi-led air strikes against Huthi rebels in Yemen. Kerry commended Saudi Arabia and other nations involved, while Tehran condemned the strikes against the Iran-backed rebels.
It was unclear if the disagreement over Yemen would affect efforts to end the 12-year standoff over Iran's nuclear program.
Neither Kerry nor Zarif responded when asked to comment on the air strikes.
Ministers from some other members of the six-power group negotiating with Iran -- which also includes Britain, France, Russia, China, and Germany -- are expected to arrive later.
France's UN ambassador, Francois Delattre, told the Security Council on March 24 that areas of disagreement include the research and development of advanced centrifuges and the lifting of U.S., European Union, and UN sanctions imposed on Iran over the years.
Centrifuges are a crucial tool for the enrichment of uranium, which can be used as fuel for a nuclear reactor or in a nuclear weapons, depending on the level of enrichment.
A senior State Department official traveling with Kerry said Washington would not rush to complete an agreement just because there was a deadline, words meant to address concerns -- raised by Israel, U.S. lawmakers, and others -- about the possibility of a deal that would not go far enough to curb Iran's activities.
The official said any framework agreement must address key aspects of a comprehensive deal that Iran and the six powers aim to conclude by June 30.
Iranian officials have suggested they could accept some kind of statement or political declaration in Lausanne, as opposed to a formal written agreement.
The last round of negotiations ended in Lausanne on March 20 after six days of discussions among representatives of Iran, the United States, and the European Union.
Officials have reported progress in the negotiations, but admit gaps remain.
Delattre said progress in the negotiations "at this point is not sufficient."
At the same UN Security Council session, Britain's Deputy Ambassador Peter Wilson warned that "we will not agree to a bad deal" and said Iran must show greater flexibility and make tough decisions in the days ahead.
Russian Ambassador Vitaly Churkin said any deal reached will be "linked to a thorough review" of the UN sanctions regime on Iran.
Iran, whose economy is hurt by restrictions on energy exports and other measures, wants all sanctions lifted swiftly.