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Brendan Smialowski, Pool, Associated Press
Iranian Foreign Minister Javad Zarif, right, and European Union High Representative Federica Mogherini wait for the start of a meeting on Iran's nuclear program with other officials from Britain, China, France, Germany, Russia and the United States at the Beau Rivage Palace Hotel in Lausanne, Switzerland Tuesday, March 31, 2015. Diplomats scrambled Tuesday to reach consensus on the outline of an Iran nuclear deal just hours ahead of a self-imposed deadline to produce an agreement.

LAUSANNE, Switzerland —

Nuclear talks between Iran and six world powers have entered into overtime, after negotiators decided to ignore a midnight Tuesday deadline and give more time to efforts to reach agreement.

Negotiations were continuing into Wednesday morning, local time. U.S. State Department spokeswoman Marie Harf says the extension was justified although "several difficult issues" still needed to be bridged.

The sides hope to reach a preliminary understanding that will allow them to enter a new phase of negotiations aiming at a final deal by June.

The U.S and its negotiating partners want to curb Iranian nuclear programs that could be used to make weapons. Iran denies such aims but is negotiating for an end to sanctions.