The UN atomic watchdog said Thursday it has completed on schedule gathering information in its probe into Iran's alleged past efforts to develop nuclear weapons.

The International Atomic Energy Agency said that, in line with a plan agreed with Iran in July, its chief Yukiya Amano will now provide a "final assessment" on the investigation by December 15.

The IAEA keeps close tabs on Iran's declared nuclear facilities to ensure that no atomic material is diverted by Iran to any covert weapons programme, an aim denied strenuously by Tehran.

Under a landmark July deal between Iran and six major powers, Iran will dramatically scale down its nuclear activities in order to render any effort to make an atomic bomb virtually impossible.

But the IAEA also wants to probe claims that at least until 2003, Iran conducted research into making nuclear weapons, including with explosives tests at the Parchin military base, something it also denies.

On July 14 -- the same day as the wider deal with major powers -- Iran and the IAEA agreed a separate "road map" agreement aimed at completing an investigation into these activities by December 15.

The plan included Iran providing the IAEA with information by August 15, which happened on schedule although the IAEA said that there remained "ambiguities" to be resolved.

Thursday's announcement by the IAEA also helps clear the way for preparations to begin for the implementation of the wider deal between Iran and major powers.

The accord, hailed as a massive diplomatic achievement after over decade of rising tensions, won final approval in Iran on Wednesday as a top panel of jurists and clerics gave it the green light.

Members of the US Congress failed in September to torpedo the deal, with President Barack Obama securing enough support in the Senate to protect the agreement.

In return for downscaling its programme, painful UN and Western sanctions on Iran are due to be lifted. Iranian officials have said this should happen by the end of 2015 or January at the latest.