Important talks between the UN atomic watchdog and Iran appeared Thursday to have failed to produce any breakthrough on a stalled probe into Tehran's alleged past efforts to develop nuclear weapons.

The International Atomic Energy Agency said in a short statement Thursday a day after a meeting in Tehran that the two sides would "continue this dialogue and agreed to meet again in the near future."

It said only that there was a "constructive exchange" about Iran answering questions about two suspect areas of activities -- eight months after an agreed August deadline to do so passed.

Iran's envoy to the IAEA, Reza Najafi, went slightly further, telling the INSA news agency late Wednesday that the two sides would "wind up the discussions (on the two issues) at the next meeting."

"We hope to reach this stage at the next session," he said, without giving a date for the next round of talks.

The Vienna watchdog conducts regular inspections of Iran's nuclear facilities and these will become tighter if world powers and Iran can finalise an outline agreement struck on April 2 by a June 30 deadline.

The European Union said Thursday that Iran and the powers -- the US, China, Russia, Britain, France and Germany -- would begin putting together the potentially historic accord in Vienna on April 22-23.

However, the IAEA also wants to investigate claims that prior to 2003, and possibly since, Iran conducted research into how to make a nuclear weapon including through explosives tests.

Iran rejects the allegations and says that the supposed bomb-making evidence, including a trove of suspicious documents reportedly found on a laptop in 2004, are forgeries provided to a gullible IAEA by Tehran's enemies.

Analysts say some of the "possible military dimensions" claims may not hold water but that Iran still has a case to answer. A probe by the IAEA made some tentative progress in early 2014 but ground to a halt last August.

The issue looks set to be a tricky one as Iran and world powers seek to nail down their final accord, with Western officials insisting sanctions on Iran will not be lifted unless Tehran cooperates with the IAEA's investigation.