Major-power foreign ministers and the UN atomic watchdog chief were set to launch a fresh two-pronged push Thursday to nail down an elusive nuclear deal with Iran in a sixth day of high-stakes diplomatic poker.

International Atomic Energy Agency chief Yukiya Amano was expected to hold talks in Tehran, seeking a way around one of the thorniest issues in the long-running talks: a stalled probe into Iran's suspected nuclear bomb development.

The foreign ministers of France, Germany, Britain and China were expected to join the main talks in Vienna, where US Secretary of State John Kerry and Iran's Mohammad Javad Zarif have been locking horns since Saturday.

EU foreign policy chief Federica Mogherini will also take part. It was unclear if Russian Foreign Minister Sergei Lavrov will also return to the Austrian capital.

"We are working very, very hard and we have some very difficult issues," Kerry said Wednesday, a day after the deadline for a deal was effectively extended to July 7.

"But we believe we're making progress and we're going to continue to work because of that," he told reporters.

Amano's visit to Tehran at Iran's invitation, where he will meet President Hassan Rouhani and other officials, is aimed at jump-starting a stalled probe into allegations of past suspicious nuclear activity by Iran.

The P5+1 powers -- the United States, Russia, China, Britain, France and Germany -- want Iran to cooperate in the investigation, saying it is a vital piece of the mooted accord.

But Iran rejects the allegations that its nuclear programme had "possible military dimensions" saying they were based on bogus intelligence provided by the CIA and Israel's Mossad.

It also baulks at the desire of the IAEA -- which it sees as biased and riddled with spies -- to visit military sites such as Parchin and interview its scientists.

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, experts say.

"It is critical that the agency has the access and information it needs to complete its investigation while respecting Iran's legitimate security concerns," said Kelsey Davenport, director for non-proliferation policy at the US-based Arms Control Association.

Amano's visit would "hopefully" pave the way to the resolution a "sticking point" in the talks, Davenport told AFP.

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

The hoped-for accord, after almost two years of negotiations and 13 years of rising tensions, focuses more on the future rather than the past, however.

Under a framework deal from April, Iran will scale back its nuclear activities with the aim of putting the development of an atomic bomb beyond its reach. return, painful sanctions would be progressively lifted.

Iran denies seeking the bomb, saying its activities are for peaceful purposes like generating electricity.

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, and on Tuesday negotiators effectively extended their deadline to get a deal until July 7.

Tough issues include the pace and timing of sanctions relief, the mechanism for their "snapback" and Iran's future development of faster nuclear equipment.