WASHINGTON — Iran has installed three-quarters of the nuclear centrifuges it needs to complete a site deep underground for the production of nuclear fuel, international inspectors reported Thursday, a finding that led the White House to warn that “the window that is open now to resolve this diplomatically will not remain open indefinitely.”

The report by the International Atomic Energy Agency, the last to be issued before the American presidential election, lays out in detail how Iran over the summer has doubled the number of centrifuges installed deep under a mountain near Qum. Iran has also, the report said, cleansed another site where the agency has said it suspects that the country has conducted explosive experiments that could be relevant to the production of a nuclear weapon.

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

The report confirmed that a recent boast by Iran’s supreme leader, Ayatollah Ali Khamenei, that the country had added nearly 1,000 centrifuges to the underground site was accurate. But it left open the question of what, exactly, Ayatollah Khamenei and other Iranian leaders intended to do with those machines, and whether, by racing ahead with construction, they were seeking negotiating advantage or trying to gain the ability to build a bomb before sanctions, sabotage or military action could stop them.