Washington urged Iran Thursday to engage with a stalled UN nuclear probe, saying it was crucial to a major accord under discussion between Iran and world powers this week in New York.

Laura Kennedy, the US representative at the International Atomic Energy Agency (IAEA), said Washington was "concerned... by the pace of progress" in the Vienna body's investigation.

"Concerns about the possible military dimensions of Iran's nuclear programme must be addressed as part of any comprehensive" deal between Iran and world powers, Kennedy said.

"Only when this happens will it be possible to have confidence that Iran's nuclear programme is and will remain exclusively peaceful," she told reporters at a meeting of the IAEA.

The IAEA regularly inspects Iran's nuclear facilities, but it also suspects that Tehran's programme had "possible military dimensions" before 2003 and possibly since.

Iran denies this, telling the IAEA in a letter in August that "most of the issues" in its probe are "mere allegations and do not merit consideration", according to the agency.

Iran agreed in May to provide information on two out of 12 suspect areas but it failed to do so by a mutually-agreed deadline of August 25. It also stopped short of proposing new measures by September 2 as requested, the IAEA says.

The comments by Kennedy came a day before the resumption of talks between Iran and the five permanent members of the UN Security Council plus Germany in New York.

Under their mooted agreement, the powers want Tehran to scale down its nuclear programme to ease fears it might get the bomb after 12 years of rising tensions.

The Islamic republic, which says its nuclear programme is to generate electricity and treat cancer patients, wants relief from painful sanctions. The deadline is November 24.