Iran and six world powers are trying to reach a nuclear deal in the Austrian capital Vienna, with just four days left to reach a deal.

Experts say the talks have reached a crucial point as Iran and the P5+1 - the US, Britain, France, China, Russia and Germany - began the final round of talks on Tuesday.

The key sticking points are the scale of Iran's uranium enrichment, and the timetable for lifting Western sanctions against the country.

In a new development, the head of the UN nuclear agency IAEA said on Thursday that Iran had yet to explain suspected atomic-bomb research.

Yukiya Amano made it clear that the IAEA was far from satisfied, saying it was not in a position to provide "credible assurance" that Iran had no undeclared nuclear material and activities.

Iran denies any intention of seeking atomic weapons, saying its nuclear programme is aimed at generating electricity.

Iran and the P5+1 group reached an interim deal last November to halt the country's nuclear programmes in exchange for an easing of some sanctions.

But after failing to narrow down gaps on key issues during six-month negotiations for a comprehensive deal, Iran and the P5+1 agreed in July to extend the nuclear talks for another four months until November 24.

The US is pushing for agreement on at least the outline of a future accord and, to this end, John Kerry, the US secretary of state, will attend the talks in Vienna on Friday.

The accord is intended to set limits on Iran's atomic activities in return for an end to international sanctions that have seriously hurt its oil-dependent economy.

No explanations

As one of the conditions, Western officials say Iran must stop blocking the IAEA investigation into allegations that Iran may have worked on designing a nuclear-armed missile, although some experts feel this should not be a deal-breaker.

"Iran has not provided any explanations that enable the agency to clarify the outstanding practical measures," Amano told the UN agency's 35-nation board of governors on Thursday.

He was referring to information Iran was supposed to have given the IAEA by late August concerning allegations of explosives tests and other activity that could indicate preparations for developing nuclear bombs.

"I call upon Iran to increase its cooperation with the agency and to provide timely access to all relevant information, documentation, sites, material and personnel," Amano said.

While the P5+1 want Iran to scale back its uranium enrichment programme - and thereby lengthen the timeline for any covert bid to assemble nuclear arms - the IAEA is investigating allegations of past research on designing an actual bomb.

"The agency is ready to accelerate resolution of all outstanding issues," Amano said.

He would present an assessment to the IAEA board "once the agency has established an understanding of the whole picture concerning issues with possible military dimensions" in Iran.

Jen Psaki, the State Department spokesperson, said Kerry would be going to Vienna from Paris to "check in" on the talks.

Source: Al Jazeera and agencies