A UN atomic watchdog team is to visit Tehran next week, state media reported Friday, amid US warnings that completion of a probe into Iran's past research is crucial for a nuclear deal.

The delegation from the International Atomic Energy Agency (IAEA) will hold talks on the "outstanding issues" in its investigation, Iran's ambassador to the Vienna-based watchdog told the official IRNA news agency.

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 bomb.

After years of Tehran insisting that it had already dealt with all of the IAEA's concerns, progress began to be made in February.

Iran provided the watchdog with explanations on one out of 12 outstanding issues, and in May promised information on two more.

However it missed deadlines in late August and early September to take the investigation forward.

That prompted Washington to voice disquiet at the pace of progress as the clock ticks down to a November 24 deadline for a comprehensive nuclear deal between Iran and the major powers.

"Concerns about the possible military dimensions of Iran's nuclear programme must be addressed as part of any comprehensive" deal, the US representative at the IAEA, Lara Kennedy, said.

A large majority of US House of Representatives members wrote to Secretary of State John Kerry on Thursday accusing Iran of "refusal" to work with the UN watchdog.

"The only reasonable conclusion for its stonewalling of international investigators is that Tehran does indeed have much to hide," the US lawmakers charged.

Iran insists its nuclear programme is entirely peaceful.