The UN atomic watchdog's board Tuesday drew a line under a long-running probe into Iran's past efforts to develop nuclear weapons, removing an important obstacle to implementing July's landmark deal with big powers.

A resolution approved by the International Atomic Energy Agency's 35-nation board of governors in Vienna "closes the board's consideration" of the "possible military dimensions" (PMD) probe and clears the way to annul previous resolutions.

Iran's envoy to the IAEA said afterwards that Tehran would now "accelerate" enacting July's accord to scale down its nuclear programme in exchange for sanctions relief, and fulfil its side of the bargain within "two to three weeks".

"We can say with certainty that the fraudulent issue of PMD is a thing of the past," Iranian Foreign Minister Mohammad Javad Zarif said in Tehran.

He said that "the purely peaceful nature of Iran's nuclear programme has once again been proven".

The IAEA keeps close tabs on Iran's declared nuclear activities, and its inspections role is set to grow under July's hard-fought deal, which defused a standoff dating back to 2002.

But the Vienna-based watchdog has also long sought to clear up allegations that until 2003, and possibly since, Iran also secretly sought to develop an actual nuclear weapon.

After stalling for many years, Iran agreed in July to cooperate with the IAEA to address the claims.

The IAEA on December 2 released a "final assessment" -- even though it did not receive all the information it sought -- concluding that some of the allegations were indeed accurate.

It said Iran conducted "a range of activities relevant to the development" of a nuclear bomb until 2009, although this work was preliminary and did not involve fissile material.

- Continued scrutiny -

Despite the findings, the six major powers -- the United States, Russia, China, Britain, France and Germany, who co-authored the IAEA resolution -- decided to move on.

Iran has a "long history of concealment, denial and deception", but the July deal is "forward-looking," the US ambassador to the IAEA, Henry Ensher, said Tuesday.

And the powers stress that Iran will remain under close IAEA scrutiny.

The July deal "prohibits the resumption of such nuclear weapons-related activities and provides comprehensive tools for deterring and detecting any renewed nuclear weapons work," US Secretary of State John Kerry said on Tuesday.

"This is not the end of the process at all," IAEA head Yukiya Amano told reporters, saying that a "legacy of mistrust between Iran and the international community" remains to be overcome.

- Implementation day -

The IAEA will also have to confirm that Iran lives up to its commitments under the July deal -- on a day to be dubbed "Implementation Day" -- which is expected in early 2016.

Under the deal Iran has pledged to slash the number of centrifuges -- which "enrich" uranium for peaceful uses but also for a bomb -- from around 19,000 to 6,104, of which 5,060 will still enrich.

Iran also has to change the design of a new nuclear reactor being built at Arak and shrink its stock of low-enriched uranium, most likely by shipping it to Russia.

The last IAEA update on November 18 said that Iran had removed some 4,500 centrifuges. However, there was no progress at Arak or on the uranium stock.

"We are about to pack up the enriched uranium and send it out. It will take just two or three days," Iran's envoy to the IAEA, Reza Najafi, told reporters.

"We are planning to (remove the Arak reactor core) in two weeks, but we will see."

Amano said however that the IAEA would still need "some weeks" to verify and confirm Iran's steps before sanctions can be lifted.