Iran failed to meet a deadline to provide answers about its controversial nuclear programme, a UN atomic watchdog report showed Friday, throwing into doubt prospects for a deal with world powers.

Tehran had agreed to provide information to allay concerns it was developing nuclear weapons, something it denies, including explosives tests that could potentially be used in a bomb.

Not answering the International Atomic Energy Agency's 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 on this possible accord 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.

To prepare the ground, Iranian and US negotiators held talks in Geneva for a second day on Friday.

Iran's lead negotiator Abbas Araqchi told the IRNA news agency that the Geneva talks were "useful and I hope they contribute to solving the disagreements, though we are still far from solving the issues".

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.

- A bigger bang -

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 IAEA suspects took place before 2003 and possibly since.

The US State Department said this week that the investigation is a "key component of what needs to be discussed" by Iran and the six powers.

The IAEA has been pressing Iran to address these claims since 2002 and in late 2011 concluded in a major report that Iran had conducted "activities relevant to the development" of a nuclear bomb.

These allegedly included large-scale explosives tests, studies on how to put a nuclear warhead into one of Iran's Shahab 3 ballistic missiles, computer models on the size of an atomic blast and preparations for a nuclear test.

Until last November, Iran had rejected all the claims out of hand, saying they were based on faulty intelligence provided by Israel's Mossad and the CIA, which it complained it was not even allowed to see.

But this February progress began to be made, with Iran promising to share information on its development of a type of detonator with various uses, such as mining, but also in a nuclear bomb. The IAEA is currently analysing this data.

In May, Tehran also agreed to exchange information on two other areas: 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 failed to provide answers on by the August 25 deadline, with the IAEA saying in the new report on Friday that they had merely "begun discussions".

The report, seen by AFP, also said that more construction work had been noticed at Iran's Parchin military base, a key site in the nuclear weapons probe, making an investigation there more difficult.

"Iran's failure to take the promised steps is a serious blow to its credibility," Mark Fitzpatrick, analyst at the International Institute for Strategic Studies, told AFP.

"If Iran doesn't take the five steps, it makes it harder for (US President Barack) Obama to persuade critics of the value of the negotiations with Iran."