The UN nuclear agency is expected to say that Iran is stalling its attempts to probe allegations that Tehran worked on nuclear weapons.
The confidential report from the International Atomic Energy Agency is significant in recording where the investigation stands for the US and five other world powers seeking a nuclear deal with Iran.
The US and its allies say no final agreement will be reached until the IAEA issues a ruling saying it is satisfied with its inquiry.
Negotiators are aiming to seal a deal on November 24 but the investigation is expected to drag on beyond that date.
Iran denies working on developing nuclear weapons.
Iran agreed in February to work with the IAEA in what was seen as a test of Tehran’s professed new willingness to reduce tensions over its nuclear programme.
Since then, the agency has sought information on alleged experiments with detonators that can be used to set off nuclear explosions, work on high-explosive charges used in nuclear blasts, and alleged studies on calculating nuclear explosive yields.
Iran denies wanting – or ever working on – nuclear arms. Since February, it has provided information only on the detonators, insisting that they were used for oil exploration or non-nuclear military purposes. The agency says interconnected information suggests they were being tested for nuclear weapons use.
“Iran has not provided any explanations that enable the agency to clarify the outstanding practical measures,” said the confidential report.