International talks on Iran’s nuclear program are due to continue in Switzerland on April 2.
Senior officials from Iran and six powers failed to reach a framework deal by a self-imposed deadline of March 31.
The talks aim at blocking Iran’s capacity to build a nuclear bomb in exchange for lifting sanctions.
U.S. Secretary of State John Kerry and German Foreign Minister Frank-Walter Steinmeier said they would stay in Lausanne at least until April 2 in an effort to seal the “political” agreement.
French Foreign Secretary Laurent Fabius returned for more talks late on April 1 after flying back to Paris the previous day because progress had been too slow.
White House spokesman Josh Earnest told reporters in Washington: “the time has come for Iran to make some decisions.”
Iran’s Foreign Minister Mohammad Javad Zarif told reporters it was the major powers who must budge, not Tehran.
Negotiators have been wrangling over the scope of uranium enrichment Iran would be allowed to conduct, where stockpiles of enriched uranium should be stored, proposed limits on Iran’s nuclear research and development, and the timing and conditions for the removal of sanctions.
The five permanent UN Security Council nations and Germany are seeking verifiable curbs on Iran’s nuclear program that ensure Tehran is not able to develop nuclear weapons.
They hope to reach a comprehensive deal by June 30.
An interim deal was reached in November 2013, but negotiators have missed two self-imposed deadlines for a comprehensive agreement since then.
Iran says its nuclear program is for purely peaceful purposes, mostly power generation, and it wants U.S., EU, and UN sanctions lifted swiftly.
Officials from both sides have said the main sticking points were the removal of the UN sanctions, the rules for reimposing them if Tehran fails to adhere to the deal, and Iranian demands for the right to unfettered research and development into advanced nuclear centrifuges after an initial 10-year period covered by the potential agreement expires.
The six powers hope to ensure that for at least the next 10 years, Iran is at least one year away from being able to produce enough fissile material for an atomic weapon, and they want Iran’s most sensitive nuclear work to be suspended for more than a decade.
With reporting by AP and Reuters
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