The UN nuclear watchdog said Thursday it will provide next week a long-awaited report on Iran's alleged efforts to develop atomic weapons, but warned its conclusions won't be "black and white".

This report on the "possible military dimensions" of Iran's nuclear programme is separate to verifying whether Iran complies with the landmark nuclear deal struck with six major powers in July.

"Next week, I expect to provide my final assessment on all past and present outstanding issues," International Atomic Energy Agency head Yukiya Amano said in Vienna.

The investigation centres on claims that mostly before 2003, but possibly since, Iran conducted research into developing nuclear weapons.

According to a major 2011 IAEA report, this included high-explosives tests at the Parchin military base and working out how to put atomic material inside the warhead of a missile.

Iran says that the claims are based on faulty intelligence provided by its enemies to a partial and gullible IAEA, and that all its nuclear activities are peaceful -- and have always been so.

Some experts have also questioned whether some of the allegations stack up, and the IAEA itself says that its information, provided largely by other countries, is only "overall" credible.

The probe was stuck for many years but this July, parallel to the wider nuclear deal with six major powers, Iran agreed to provide information and access to sites including Parchin.

Under that wider accord, Iran agreed to downsize its nuclear programme in order to make any dash to make nuclear weapons all but impossible. In exchange sanctions will be lifted.

But clearing up the IAEA's probe into what Iran may have done in the past is essential for regaining trust between Iran and the international community, the six powers say.

- Jigsaw puzzle -

Amano said on Monday that although IAEA now has a "better understanding" of Iran's past activities, his report next week will not be a "black and white assessment."

"This is like a jigsaw puzzle. We have pieces, we have a better understanding and we will analyse and provide an assessment," he told reporters.

"This is not an issue which can be answered 'yes' or 'no'."

The final report -- which diplomats say will be released next Tuesday or Wednesday -- is expected to be presented to the IAEA's board of governors on December 15.

This board -- which comprises 35 out of the IAEA's 167 members including all six major powers -- will then decide whether to "close" the matter, Amano said.

Separately, the Vienna-based IAEA is also tasked with confirming that Iran complies with the wider deal with world powers.

Iran's deputy foreign minister said this week that all necessary initial steps will be completed in early January.

This will allow "implementation day" for the deal, when sanctions begin to be lifted, to happen then.

Some experts believe however that this timetable may be overly ambitious.

Amano said Monday that so far, Iran has removed "quite a large number of centrifuges at quite a high pace" but that it was difficult to predict how long all the steps will take.