Iran's boycott of a rare UN atomic agency forum on creating a Middle East free of nuclear weapons was "not helpful to their cause", the chairman of the event said Tuesday.

"I think it is to be regretted that Iran did not participate," Norwegian ambassador Jan Petersen told reporters at the end of the two-day forum at the Vienna-based International Atomic Energy Agency (IAEA).

"I would certainly have welcomed their participation in this forum. It was the only country of the region not participating ... I can't see that it is in any way helpful to their cause (not to attend), but it is their decision."

Iran snubbed the event after the IAEA board on Friday passed a resolution of "deep and increasing concern" about Tehran's suspected nuclear weapons drive following a damning report by the watchdog the week before.

Britain, Canada and the United States on Monday announced additional restrictions on Iran's financial, petrochemical and energy sectors. The European Union is also expected to tighten the screws.

"Of course after the board meeting of last week they have quite a lot of questions to answer, and they have one more question to answer now, and that is why they didn't participate," Petersen said.

Iran's IAEA ambassador, Ali Asghar Soltanieh, said the forum, attended by all other countries in the region including Israel and Syria, was "useless" as long as Israel continued to deny having a nuclear arsenal.

Petersen said that the event, which comes ahead of a 2012 NPT (nuclear non-proliferation treaty) conference in Finland, was "a small positive step, but there is a very long way ahead".

The main aim of the forum was learning from the experiences of other so-called nuclear-weapon-free zones (NWFZ) such as Southeast Asia, Africa and Central Asia, he said.

The Middle East is a considerably more volatile region, however, marked by animosity between states and rocked this year by "Arab Spring" popular uprisings toppling regimes in a string of countries.

Israel is widely believed to possess nuclear weapons but has never confirmed it. Unlike Iran it is not a signatory of the NPT and therefore not subject to IAEA inspections -- as highlighted by several Arab states at the forum.

Participants said the atmosphere was however less confrontational than previous IAEA events that have degenerated into Arab-Israeli slanging matches, most notably the annual conferences of the agency's roughly 150 member states.

IAEA chief Yukiya Amano said in closing remarks that the event had shown it was "possible to have a constructive dialogue related to the establishment of a NWFZ, despite the complexity of the issue and differences of view."

"As representatives of existing NWFZs explained, it can take many years -- even decades -- but mistrust between key parties should be overcome in time and should be replaced by mutual confidence and cooperation.

"This requires a combination of political will and commitment, dialogue, flexibility and an innovative and incremental approach," Amano said.