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 is 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 America and the 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 America 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 airshafts 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 America or Israel.

An inspection team from the U.N. nuclear watchdog, the International Atomic Energy Agency, will begin work today in Natanz. The team will check electrical work under way and verify that Iran is complying with the suspension in advance of the IAEA board meeting next week in Vienna, Austria.

The agency has been investigating Iran’s nuclear program since an exile group disclosed the existence of Natanz in August 2002, slowly unraveling an ambitious Iranian effort that was kept secret for nearly two decades. While questions remain, the IAEA says it has found no evidence of a weapons program.

Two intelligence officials and a nuclear expert, all from a government opposed to Iran’s nuclear efforts, said they had developed “very solid information” about plans to install 54,000 centrifuges at Natanz. They said up to two-thirds of them would be the advanced model, known as the P-2.

They conceded that they were uncertain about the key issue of when Iran would build and install the machines and that they did not have hard evidence that Iran was currently manufacturing P-2s.

Tehran told the IAEA last year that it had stopped all research and development on P-2s. If Iran is building the advanced centrifuges, it would violate its agreements with the three European nations and the international agency, diplomats said.

In separate interviews, diplomats close to the IAEA said that, while it is likely Natanz will eventually house P-2s, they have no information that Iran is working on the machines.

“Their having made some planning should not be overly surprising,” said a Western diplomat in Vienna. “However, if there were production going on it would be a breach of the suspension.”

A senior Iranian official dismissed the idea that Iran was working on P-2s now, but he said that Natanz was designed to accommodate either the P-2 or the less advanced P-1.

A senior IAEA official is expected to provide an update on Iran’s compliance when the agency board meets next week. The Western diplomat in Vienna suggested that the release of information about the P-2s was timed to fuel doubts about Iran.

“The question has been: Do they already have the P-2 developed and demonstrated?” said David Albright, a former IAEA inspector who is head of the Institute for Science and International Security in Washington. “My understanding is that there is not much progress being made on this [question] by the IAEA.”

Iran began building P-1 centrifuges several years ago and told the Europeans in April that it would install 3,000 of them at Natanz. The number is far more than planned for a nearby pilot plant and could turn out enough enriched uranium for one or two bombs in a year.

The Bush administration recently pushed back the date at which it believes Iran could have an atomic weapon if it resumes enrichment activities.

The director of the Pentagon’s Defense Intelligence Agency, Vice Admiral Lowell E. Jacoby, told a Senate committee in March that Iran is not expected to be able to produce a nuclear weapon before early in the next decade, several years after earlier estimates.

Mr. Albright said he was told the assessment was shared throughout the American intelligence community. But Israeli intelligence estimates that Iran could have a nuclear weapon within two years or less of resuming enrichment, a much shorter period.