"The Secretary will gauge the extent of Iran's willingness to commit to credible and verifiable steps that would back up its public statements about the peaceful nature of its nuclear program," State Department deputy spokeswoman Marie Harf said in a statement.
"The Secretary will see if progress can be made on the issues where significant gaps remain and assess Iran's willingness to make a set of critical choices at the negotiating table," she added. The six powers have a July 20 deadline for a deal, although an extension is theoretically possible.
Schedules permitting, foreign ministers from Britain, France, Germany, Russia and China will join their U.S. and Iranian counterparts, along with EU foreign policy chief Catherine Ashton, Harf said.
