The top U.S. diplomat and his Iranian counterpart ended a nearly two-hour meeting in Switzerland Sunday.
U.S. Secretary of State John Kerry tweeted a picture of his talks with Mohammad Javad Zarif. They joined talks already under way in Geneva between Iran's Atomic Energy Organization chief Ali Akbar Salehi and U.S. Energy Secretary Ernest Moniz.
The talks, which will resume Monday, are an attempt to reach a deal on Iran's nuclear program by a March 31 deadline.
The round of discussions between the U.S. and Iran's top diplomats and energy officials is aimed at helping resolve ongoing technical disputes standing in the way of an agreement between Iran and the six world powers known as the P5+1.
A deal between Iran and the group - made up of the U.S., Britain, China, France, Germany and Russia - would curb Iran's nuclear activities in exchange for sanctions relief.
Kerry, who met with the British foreign secretary in London Saturday, said the P5+1 remains united on Iran.
"There is absolutely no divergence whatsoever in what we believe is necessary for Iran to prove that its nuclear program is going to be peaceful in the future," he said.
The latest talks come as a new report from the United Nations nuclear agency says it remains concerned about the possible existence of “undisclosed nuclear-related activities” in Iran that could include work linked to the development of a nuclear payload for a missile.
But the International Atomic Energy Agency (IAEA) said Iran is in compliance with some provisions of a nuclear agreement.
Iranian state media reports quote Iran's IAEA ambassador, Reza Najafi, as saying the findings show his country’s “full transparency” and the peaceful nature of its program.
Some material for this report came from AFP and Reuters.