The head of the UN atomic watchdog hit out Monday at Iran over its lack of cooperation with a probe into its nuclear programme, three weeks after Tehran missed a deadline to provide important information.

"In order to resolve all outstanding issues, it is very important that Iran continues to implement, in a timely manner, all practical measures agreed" to improve transparency, International Atomic Energy Agency chief Yukiya Amano said.

This was despite Iran's "statement of a firm, high-level commitment... and its stated willingness to accelerate the resolution of all outstanding issues," Amano told the IAEA's board of governors, according to the text of his speech.

The IAEA regularly inspects Iran's nuclear facilities, but it also wants Iran to respond to allegations that its programme in the past had "possible military dimensions" -- in other words that it researched how to build a nuclear bomb.

After years of Tehran rejecting these claims, progress began to be made in February, with Iran providing the IAEA with explanations on one out of 12 suspect areas and in May promising information on two more.

Iran however failed to do so by a mutually agreed August 25 deadline. Amano said Monday merely that the IAEA and Tehran has "begun discussions".

Iran also failed to propose new measures by September 2 as requested, he said.

Iran's envoy to the IAEA, Reza Najafi, said last week that Iran had informed the agency that the August 25 deadline would not be met and that further talks would be held in Tehran in late September.

Not answering the IAEA's long-standing questions -- including suspect activities on a military base in Parchin -- could harm the chances of reaching a historic deal between Iran and world powers, experts say.

That accord would be focused on the future of Iran's declared nuclear facilities, which Tehran says are for peaceful purposes but which Western countries fear could one day enable it to build a bomb.

New talks on this mooted agreement between Iran and the five permanent members of the UN Security Council plus Germany are due to resume in New York on Thursday ahead of a November 24 deadline.

Experts and diplomats believe that, at the current pace, the IAEA's probe will take roughly one to two years to complete -- assuming Tehran cooperates.

"At this stage I cannot say how long it will take to clarify the 12 areas.... However this is not an endless process," Amano told a news conference in Vienna.

"With the cooperation of Iran we can clarify these issues within a reasonable timeline."