Istanbul, Turkey ? Iran plans to install tens of thousands of advanced centrifuges at its huge underground nuclear plant near the central city of Natanz, which eventually would enable it to enrich uranium nearly twice as fast as anticipated, according to Western intelligence officials.

Iran’s timetable remains unknown, but the officials said preparatory work was under way at the plant and the decision to rely on the superior type of centrifuge suggests Iran could manufacture fissile material for a possible weapon sooner than expected.

Diplomats with knowledge of Iran’s nuclear program said they could not confirm the information, but Tehran said last year that it intended to use the updated centrifuges at some point in the future.

Iran insists its nuclear program is for peaceful purposes, but the United States and European Union fear Tehran intends to build atomic weapons in violation of the Nuclear Non-Proliferation Treaty.

Stopping Iran from mastering uranium enrichment is the central goal of the United States and the European Union. They have threatened to refer Iran to the U.N. Security Council if Tehran abandons an agreement reached with three European governments last November to suspend enrichment activities.

The concern centers on the possibility that, after developing sufficient enrichment capabilities, Iran could more readily shift production from low-level enriched uranium for nuclear reactors to high levels for weapons, either secretly or after withdrawing from the nonproliferation treaty.

Iranian officials pledged Sunday to extend the voluntary suspension of enrichment until the end of July as part of negotiations with Germany, France and Britain over the disputed nuclear program. But Tehran has said that the suspension is both voluntary and temporary and that it intends to eventually produce fuel for civilian reactors.

The complex at Natanz, about 150 miles south of Tehran, is the heart of Iran’s enrichment effort. Plans call for more than 50,000 centrifuges to be installed in two vast underground halls where they could produce large quantities of enriched uranium, the Western intelligence officials said.

Earlier this year, Iran finished covering the main plant with 25 feet of concrete and an additional layer of earth. Satellite photos show that the entrance to the underground plant and two large air shafts were concealed by what appear to be dummy buildings.

Journalists taken on a government tour of Natanz in March reported that the 1,100-acre site is ringed by at least 10 anti-aircraft batteries. Iranian officials said the missiles and underground facilities were prompted by concerns over possible attacks by the United States or Israel.