VIENNA – Iran has effectively shut down a probe of a site suspected of being used for work on nuclear weapons development while doubling the number of machines it could use to make the core of nuclear warheads at an underground bunker safe from airborne attack, the U.N. nuclear agency said in a report Thursday.

Senior diplomats familiar with the International Atomic Energy Agency's latest report called both developments troubling, while cautioning that it was unclear how many of the more than 2,000 centrifuges now installed at the Fordo site were operational. They noted that agency inspectors did not have access to check their inner workings.

They also said it was unclear what the ultimate use of the machines would be, if and when they are turned on. Iran is enriching uranium at a level that could be used for reactor fuel as well as grades closer to the level used for nuclear warheads, and the officials said Iranian officials had not said what level the centrifuges now installed but not working would be used for.