A missile is seen inside an underground missile base for Iran's Islamic Revolutionary Guards Corps (IRGC) Aerospace Force at an undisclosed location in this undated handout photo courtesy of Fars News. REUTERS/farsnews.com/Handout via Reuters

By Francois Murphy and Shadia Nasralla

VIENNA (Reuters) - A U.N. report on whether Iran has in the past carried out work related to nuclear weapons will not reach a definitive conclusion on the subject, the chief of the U.N. nuclear agency said on Thursday.

The report by the International Atomic Energy Agency (IAEA) is required under a deal between world powers and Iran which provides for a lifting of sanctions against the Islamic Republic in exchange for restrictions on its nuclear program.

A clear verdict that weapons-related work occurred would have made it difficult for the deal to proceed, but diplomats have said for weeks that they do not expect the report to be clear-cut, which IAEA chief Yukiya Amano confirmed on Thursday.

"The report will not be black and white," Amano told a news conference when asked about the final assessment of possible military dimensions, or PMD, file which is due next week.

In November 2011, the IAEA issued an annex to a regular report on Iran, listing twelve issues which could indicate Iran might have had a nuclear weapons program.

Iran denies these allegations, but the IAEA, which says it takes no information at face value, has described the intelligence from its member states which formed the basis of the PMD file as "overall credible".

The annex talks about specific technology to synchronize detonators, create neutron bursts to induce fissile chain reactions as needed for a nuclear bomb as well as engineering studies into missile payloads and their behavior when launched.

The investigation leading to the report, which is aiming to answer questions on these and other points that have gone unanswered for years, had given the agency a better understanding of the "whole picture" of Iran's past nuclear activities, Amano said, but gaps remained.

"What I can now say is that this is an issue that cannot be answered by 'yes' and 'no'," Amano said.

Technically, Iran cleared the crucial hurdle towards sanctions relief with respect to the PMD file on Oct. 15, when it provided the agency with enough data to allow the written assessment.

Still, the report will be seized upon both by opponents and supporters of the July deal, which is designed to eventually lift sanctions.

When asked about the likely response, Amano said the PMD document would be "factual" and that any decision on further investigation was in the hands of the IAEA's member states.

Iran's envoy to the IAEA said Iran would only fulfil its promises to disable its sensitive Arak reactor and to reduce its enriched uranium stockpile if the PMD file was formally closed at the next IAEA board of governors meeting on Dec.15.

"We're confident that by (Dec. 15), when (Amano) provides the final assessment, the member states would take the right decision on concluding the issue," Reza Najafi said.

(Editing by Louise Ireland and Angus MacSwan)