Iran obtained detailed instructions on how to set up the complicated process of enriching uranium, which can used to make nuclear arms, from the black market network run by a Pakistani scientist, the U.N. atomic monitoring agency said Friday.

In a confidential report, the International Atomic Energy Agency also said Iran was not giving inspectors access to a sensitive site that could be used to store equipment indicating whether the military is running a secret nuclear program.

The five-page report seen by The Associated Press was prepared for Thursday's meeting of the IAEA's 35-nation board, which could decide to refer Tehran to the U.N. Security Council for possible sanctions for violating an international nuclear arms control treaty.

Most board nations are concerned that Iran has resumed uranium conversion — a precursor to enrichment — and has refused to meet all IAEA requests about a nuclear program that was clandestine for nearly 20 years until discovered three years ago.

The report said Iran had handed over documents revealing detailed instructions on setting up uranium enrichment that it obtained from the black market network of Abdul Qadeer Khan. The scientist, considered the father of Pakistan's nuclear program, has acknowledged selling secrets to Iran, Libya and North Korea.

The designs obtained from the black market show how to cast enriched, natural and depleted uranium metal into a spherical form, the agency said. Diplomats accredited to the agency said that could indicate a design for the core of a nuclear weapon.

Agency officials refused to comment on the implications of the finding. The report said Iran insisted it had not asked for the designs but was given them anyway by members of the nuclear network — something an official close to the agency said the IAEA was still investigating.

The diplomats and officials spoke on condition of anonymity because of the sensitivity of the matter.

The IAEA report also said more transparency by Tehran was "indispensable and overdue" as agency inspectors try to determine if Iran's military secretly ran its own nuclear program parallel to a civilian one.

Inspectors needed access both to more details on Iran's enrichment activities and a site where it is believed to be warehousing equipment that could be used in a weapons program, the document said.

"There still remain issues to be resolved" in connection with whether the military was supplied with centrifuge technology in the mid-1990s and then conducted secret enrichment activities between 1995 and 2002, it said.

The report said the key outstanding issues concerning Iran's nuclear program include whether the military was involved in enrichment, access to the military site where the "dual use" equipment was believed held and greater access to individuals involved in the enrichment program.

"Transparency measures should include the provision of information and documentation related to the procurement of dual-use equipment and permitting visits to relevant military-owned workshops and R&D locations thought run by the military," the report said.

The agency is "still awaiting additional visits," both to the military site at Lavisan-Shian, just outside Tehran, and to Parchin, which IAEA inspectors visited for the second time a few weeks ago.

"These should include interviews on the acquisition of certain dual-use materials and equipment, and the taking of environmental samples from the above locations," the IAEA said.