Iran has installed three-quarters of the nuclear centrifuges needed for a deep-underground site producing nuclear fuel, doubling their numbers, international inspectors reported Thursday.

The International Atomic Energy Agency's report details how Iran over the summer has doubled the number of centrifuges installed at Fordow, deep under a mountain near the holy city of Qom, the New York Times reported. Iran has also, the IAEA said, cleansed another site where it suspects explosive experiments linked to the production of a nuclear weapon have been conducted.

Based on satellite photographs, the agency said the cleanup had been so extensive that it would "significantly hamper" the ability of inspectors to determine what kind of work had taken place there.

The report confirmed that a recent boast by Supreme Leader Ayatollah Ali Khamenei, that Iran had added nearly 1,000 centrifuges to the underground site was accurate. But it left open the question of what the purpose of those machines was and whether, by racing ahead with construction, the Iranians were seeking negotiating advantage or trying to gain the ability to build a bomb before sanctions, sabotage or military action could stop them. 

The IAEA also said buildings had been demolished and earth removed at a military site it wants to inspect, in what Western diplomats see as a determined effort by Tehran to clean up any evidence of illicit nuclear-linked tests, Reuters reported.

These "extensive activities" at the Parchin complex, the Vienna-based U.N. agency added, would significantly hamper its investigation there, if and when inspectors are allowed access.

The building, which the IAEA believes is housing a steel chamber for explosives tests, has now been "shrouded," the report said, in a possible attempt to hide it from satellite cameras.

In another apparent sign of stonewalling of the IAEA's inquiry, it said "no concrete results" had been achieved in a series of high-profile meetings with Iran this year aimed at allaying concern about its nuclear research.