Iran has prepared more than 1,000 advanced uranium enriching machines for startup the U.N.’s nuclear agency said Wednesday, a move that is likely to raise concerns among countries who accuse Tehran of wanting to harness enrichment for the production of atomic arms.

At the same time, the report by the International Atomic Energy Agency said Iran now has pushed back the time frame for the operation of a reactor that Iran’s critics fear could be used to make plutonium, which — like enriched uranium — can be used for the fissile core of nuclear weapons.

The report also confirmed that the IAEA and Iranian experts have agreed to restart talks focused on the agency’s attempts to probe suspicions that Tehran worked on atomic weapons, in what would be first such meeting since Iran’s hard-line president was replaced by a more moderate successor. News of the planned Sept. 27 meeting was first revealed by diplomats earlier Wednesday.

Iran denies any interest in nuclear weapons, insisting that both enrichment and the reactor are meant for peaceful purposes, such as production of energy and medical and scientific research. Since 2006, it has shrugged off numerous U.N. Security Council and other international sanctions meant to curb its nuclear activities, as well as incentives offered during international negotiations and aimed at the same goal.

The confidential report was released Wednesday to the agency’s 35 board member nations and the Security Council. It said Iran had installed about 300 more of its advanced centrifuges since the last report in May, for a total of 1,008, and had put all of them under vacuum.

Such a move is normally one of the last steps before the machines start spinning uranium gas into the material that can be used either as reactor fuel or as the core of nuclear warheads, depending on its enrichment level.

The report also said Iran had installed more of its older-generation centrifuges to bring up their number to more than 15,000, with most of them running. But most concerns are likely to be generated by the pre-start up work on the high-tech IR2-m centrifuges because they are three to four times more effective than the older IR-1 machines.

In addition to putting the existing IR2-ms under vacuum, pre-installation work was continuing for about 2,000 additional advanced centrifuges, the report said.

Commenting on the report’s findings, U.S. State Department spokeswoman Marie Harf said Washington remains “concerned about Iran’s continued expansion of its enrichment capability.”

Summarizing the progress of construction of the plutonium-producing reactor at Arak, in central Iran, the IAEA noted some advances. At the same time it cited an Iranian letter telling the agency that because of unspecified delays, the startup date was “not achievable, so it cannot be the first quarter of 2014.”

That target date, previously cited by the Islamic Republic, has been described as unrealistic by nuclear experts who say the reactor is unlikely to be operating before sometime in 2015 at the earliest.

With its stockpile of enriched uranium at thousands of pounds and growing, Iran theoretically has enough material to make several nuclear weapons. But most of the material is enriched only to fuel grade and any move to turn it into uranium for weapons is difficult and quickly detectable.

A smaller cache is enriched to higher levels that make it more easily convertible. But the report noted that this more sensitive supply remained below the amount need to convert into the 50 to 55 pounds)of high-enriched uranium needed to make one weapon, with Iran continuing to turn most of what it makes into a form that is difficult to rework into weapons use.