Iran and six major powers reached a framework nuclear deal on April 2 in Lausanne, Switzerland, and are seeking to strike a final agreement by June 30 under which Iran would restrain its nuclear program in exchange for relief from economic sanctions.
"As we expected after Lausanne, the next portion of this process will be pretty tough because we will be getting down to the details," the official told reporters as negotiators from the seven prepared to converge on Vienna for a new round.
The United States, which is negotiating along with Britain, China, France, Germany and Russia, suspects Iran is trying to develop a nuclear weapons capability. Iran denies that, saying its program is for peaceful uses like making medical isotopes.
The U.S. official said the sides were having “detailed discussions” about access to Iran’s military sites.
On Tuesday, the European Union said its political director, Helga Schmid, and negotiators Abbas Araqchi and Majid Takht-Ravanchi from Iran would meet in Vienna on Wednesday and would be joined later in the week by the other negotiators.
"Everyone in the room is focused on June 30. It doesn’t help any of us to delay difficult decisions," the U.S. official said.