Talks between Iran and major powers towards a historic nuclear deal are set to intensify Thursday with the head of the UN atomic watchdog due in Tehran, seeking to resolve an impasse over inspecting suspect military sites.
"We are working very, very hard and we have some very difficult issues," US Secretary of State John Kerry said Wednesday after meeting his Iranian counterpart Mohammad Javad Zarif once again.
"But we believe we're making progress and we're going to continue to work because of that," he told reporters in Vienna, which is hosting the final stretch of almost two years of negotiations.
Kerry later met with German Foreign Minister Frank-Walter Steinmeier, who returned to the city for the second time in a matter of days.
France's Laurent Fabius is expected back Thursday too, while China's Foreign Minister Wang Yi will arrive for the first time during this round. EU foreign policy chief Federica Mogherini will also return to the Austrian capital after the deadline for a deal was extended to July 7.
Iran and the P5+1 group -- the United States, Russia, China, Britain, France and Germany -- are trying to nail down a historic agreement ending a 13-year standoff over Iran's nuclear ambitions.
Under a framework deal agreed in April, Iran will scale back its nuclear programme with the aim of putting an atomic bomb beyond its reach.
In return, painful sanctions imposed on Iran would be progressively lifted.
Iran said Wednesday it had recovered 13 tonnes of its gold reserves frozen in South Africa under international sanctions.
Central bank chief Valiollah Seif, quoted by the IRNA news agency, said the operation was the result of a compromise struck on the sidelines of the Vienna talks.
Iran has long denied seeking to develop a nuclear bomb, and one of the main sticking points has proved to be access to sensitive Iranian military sites.
The International Atomic Energy Agency (IAEA) wants to probe allegations that before 2003 and perhaps since, Iran's nuclear programme had "possible military dimensions," meaning it was conducting nuclear weapons research.
Iranian media said IAEA chief Yukiya Amano would meet Thursday with President Hassan Rouhani at Iran's invitation.
They would "talk about past activities and receive Iran's suggestions on how to resolve the differences," the Iranian news agency IRNA said.
Another Iranian agency, ISNA, said the country's leaders would offer "suggested solutions" to the deadlock as they seek to flesh out the framework deal into a comprehensive accord.
An accord would be a rare diplomatic success in the Middle East, potentially putting Iran on the road to better international relations -- a prospect that alarms rivals Israel and Saudi Arabia.
The IAEA said Amano's visit was to "accelerate the resolution of all outstanding issues related to Iran's nuclear programme, including clarification of possible military dimensions".
Other tough issues include the pace and timing of sanctions relief, the mechanism for their "snapback" and Iran's future development of faster nuclear equipment.
The IAEA already keeps close tabs on Iran, with its inspectors and monitoring equipment making sure that all nuclear material is accounted for and that Iran's facilities are being used for exclusively peaceful purposes.
Under any accord, it will have greater inspections rights and will have to verify that Iran is complying with the deal before any sanctions are lifted.
The P5+1 also want the IAEA in the future to be able to go anywhere it likes in order to probe allegations of suspicious activity.
One possible compromise might be strictly controlled "managed access" visits that reassure Iran that IAEA staff are not spying on its military facilities under the guise of inspections.