Intense talks between Iran and world powers ramped up Wednesday with the chief of the UN atomic watchdog due in Tehran seeking to resolve an impasse over a nuclear bomb probe.

"We are working very, very hard and we have some very difficult issues," US Secretary of State John Kerry said after a meeting with his Iranian counterpart Mohammad Javad Zarif.

"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.

The foreign ministers of France and China, Laurent Fabius and Wang Yi, are expected in Vienna Thursday along with EU foreign policy chief Federica Mogherini 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 massively scale down its nuclear programme with the aim of putting the development of an atomic bomb beyond its reach.

In return, painful sanctions imposed on Iran would be progressively lifted.

But 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" -- in other words, that it conducted nuclear weapons research.

Iranian media said IAEA chief Yukiya Amano would visit Tehran for talks on Thursday with President Hassan Rouhani at Iran's invitation.

"The goal of Yukiya Amano's trip is to talk about past activities and receive Iran's suggestions on how to resolve the differences," the Iranian news agency IRNA said.

- Solutions? -

Another Iranian agency, ISNA, said the country's leaders would offer "suggested solutions" to the deadlock.

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.

But finalising the framework agreement from Lausanne, Switzerland has proved difficult.

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.

Russian Foreign Minister Sergei Lavrov said Tuesday that despite the extension he believed an accord was "within reach".

"There remain questions, mostly regarding procedural issues rather than technical," Lavrov told Russian television after talks with Kerry.

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.

Iran says its programme is peaceful and has always been so. An IAEA probe has been practically stalled since mid-2014.

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.
Source: AFP