"Iran had warned the International Atomic Energy Agency that because of the complexity of the issues, implementing all five points by August 25 was not possible," said Behrouz Kamalvandi, spokesman of the Atomic Energy Organisation of Iran, quoted by state news agency IRNA.
"The IAEA was aware of this," he said.
"We do not have any commitment on a date… But we have always said we will try to deliver all the clarifications as soon as possible."
The Vienna-based IAEA said Friday Iran had failed to meet an August 25 deadline to provide information on five points to allay concerns it was developing nuclear weapons, something it denies.
IAEA building in Vienna. Photo: Paul Gillingwater
Not answering the long-standing questions over the allegations could harm the chances of a potentially historic deal between Iran and world powers focused on Tehran's current activities.
New talks between Iran and the five permanent members of the UN Security Council plus Germany are due to resume in New York on September 18 ahead of a November 24 deadline.
The mooted deal, after a decade of rising tensions, would kill off fears that Iran might use its nuclear facilities — which it says are for peaceful purposes — to develop atomic weapons.
To do this the United States, China, Russia, Britain, France and Germany want Iran to scale back its nuclear programme in exchange for relief from painful sanctions.
Vital to the deal is the IAEA's probe into what it calls the "possible military dimensions" of Iran's atomic programme — work on developing a nuclear weapon that the watchdog suspects took place before 2003 and possibly since.
In May, Tehran agreed to exchange information on large-scale tests of explosives that could be used in a nuclear bomb, and calculations on the size of a nuclear explosion.
It is these two areas that Iran has so far failed to provide answers, with the IAEA saying Friday that the two sides had merely "begun discussions".