"The gap still exists, differences exist, and all parties are negotiating with seriousness and determination, but we haven't found solutions to key questions," Deputy Foreign Minister Abbas Araghchi said.
He was speaking after three hours of talks on Sunday night between US Secretary of State John Kerry and Energy Secretary Ernest Moniz and their Iranian counterparts Mohammad Javad Zarif and Ali Akbar Salehi.
The US and Iranian officials began their talks on Friday.
Parallel negotiations were held on Sunday between Tehran and senior negotiators from the so-called P5+1 group – UN Security Council permanent members Britain, China, France, Russia and the United States plus Germany.
"In many areas, the negotiations have addressed the details... In some cases, solutions were found and the time has come for political decisions," said Araghchi, quoted by state television.
"For this reason, contacts at the highest level between the two parties are needed," he added.
Kerry and Zarif are holding a second day of discussions on Monday as the clock ticks down to a March 31 target for a framework accord.
Their meetings are a bid to smooth the way towards a long-elusive nuclear deal that would prevent Iran from developing a nuclear bomb in return for an easing of punishing international economic sanctions.
Iran denies its nuclear programme has military objectives.
"Today, no one can pretend that Iran's nuclear programme is not peaceful," President Hassan Rouhani told science ministers from Non-Aligned Movement nations gathered in Tehran.
"The Iranian people have suffered the worst pressure during the past dozen years for having sought to realise their rights in the fields of science and nuclear technology," he said in remarks posted on the government's website.
A key stumbling block in any final deal between Iran and world powers is thought to be the amount of uranium Tehran would be allowed to enrich, and the number and type of centrifuges it can retain.

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