The next round of Iran nuclear negotiations will be "pretty tough," a senior U.S. official said on Wednesday ahead of new talks including top diplomats from the seven countries involved.

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.

Among the key issues yet to be resolved are the sequencing of any sanctions relief for Iran as well as the monitoring and verification measures to ensure Iran is not secretly violating any agreement that might be reached.

The U.S. official said the sides were having “detailed discussions” about access to Iran’s military sites.

"We know there will not be an agreement until we can resolve that the IAEA will be able to verify whatever way is appropriate, whatever is necessary for this agreement, and that will include having managed access to a variety of sites and places in order to get this deal done," the official said.

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.

By Reuters