Iran and six world powers resumed negotiations on the country's nuclear program on Thursday, the Iranian state news agency IRNA said, seeking to overcome remaining differences with a self-imposed June 30 deadline looming to end a 12-year standoff.

A framework accord was reached between Iran, the United States, France, Britain, Germany, Russia and China on April 2, but several major substantive disputes remain to be resolved, including access for U.N. nuclear inspectors to Iranian military sites and the pace and timing of sanctions relief for Tehran.

"We have a few weeks and hope to reach a final deal by the June 30 deadline or even sooner," IRNA quoted Iranian deputy Foreign Minister Abbas Araqchi as saying after his arrival in Vienna to resume the talks.

"There has been progress but still we have a difficult way ahead of us."

Iranian Supreme Leader Ayatollah Ali Khamenei, who last the say on all matters of state, and Iran's top military commanders have flatly rejected access to military sites under any deal that would curb the Iranian nuclear program.

"Inspection and access to non-nuclear and military sites will not be accepted by Iran. Controlled and managed access does not mean inspection," Araqchi said. "We are trying to set some rules for managed access to non-nuclear sites."

France said last week it was ready to block any final nuclear settlement unless Tehran provided inspectors access to all installations, including military sites.

Iranian and French officials have hinted at the possibility of an extension of the deadline, but the United States said an agreement can be reached by June 30.

Other lingering issues to be resolved include monitoring and verification measures to ensure Iran could not pursue a clandestine nuclear weapons program.

The Islamic Republic denies any ambition to develop a nuclear arms capability, saying it has been enriching uranium only for peaceful electricity and medical isotopes.