Secretary of State John Kerry joined other top diplomats from around the world in Geneva on Saturday to continue negotiations on Iran's nuclear program.
U.S. negotiators and their counterparts from the United Kingdom, France, Germany, Russia and China have been meeting with the Iranians since Wednesday in an effort to strike an interim deal to delay Iran's nuclear program while a larger deal is worked out that would prevent the Islamic republic from obtaining nuclear weapons.
Britain's Foreign Secretary William Hague spoke of "very difficult negotiations, saying "narrow gaps" remain on the same issues that blocked agreement at the last round earlier this month.
"We're not here because things are necessarily finished," Hague told reporters. "We're here because they're difficult, and they remain difficult."
Kerry decided to go after consulting with European Union representative Catherine Ashton and his negotiating team in Geneva, State Department spokeswoman Jen Psaki said in a statement Friday.
It was unclear whether negotiations would continue Sunday. Psaki said Kerry still planned to travel to London on Sunday for meetings on other Middle East issues.
The current talks, which are in their third round, aim to take what Kerry has called "a first step" that would delay Iran's progress toward the ability to produce nuclear weapons for six months while a more comprehensive agreement is worked out.
The last round of talks broke down over French concerns about the status of Iran's heavy water power plant under construction in Arak, and over Iran's demand that any agreement recognize the production of nuclear fuel as Iran's sovereign right. The USA does not recognize such a right.
World powers have imposed crippling economic sanctions on Iran to persuade it to suspend production of nuclear fuel in accordance with United Nations Security Council resolutions, and to prove its nuclear program is peaceful in compliance with the Non-Proliferation Treaty, which Iran has signed.
Iran, which seeks to have the sanctions lifted, says its nuclear program has peaceful aims. But the U.N.'s nuclear watchdog, the International Atomic Energy Agency, has reported over the years multiple discoveries of secret Iranian nuclear sites, work on nuclear detonators and implosion devices with assistance from foreign scientists, documents describing safety arrangements for a nuclear test, and plans for a spherical payload for Iranian missile.
The United States, Israel and European countries have said they suspect Iran of developing a nuclear weapons program. President Obama has said the USA will prevent Iran from obtaining a nuclear weapon and Israel has threatened to use military force to prevent what it considers an existential threat.