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The diplomat, based in Vienna which is the headquarters of the International Atomic Energy Agency, cited a letter from Iranian officials to the I.A.E.A. saying it wants to upgrade its main enrichment plant at Natanz. The upgrade could speed up enrichment by as much as two or three times, the diplomat said, requesting anonymity in light of the confidential nature of the Iranian note.

The United States and its allies accuse Tehran of seeking the technology for nuclear weapons but Iran says it wants to use enriched uranium purely for civilian and peaceful purposes.

The disclosure came at a time of high regional tension, a day after American officials said Israeli warplanes struck deep inside Syrian territory on Wednesday. The American officials said they believed the target was a convoy carrying sophisticated antiaircraft weaponry on the outskirts of Damascus that was intended for the Hezbollah Shiite militia in Lebanon.

Iran is a close regional ally of both Syria and Hezbollah. While an accelerated Iranian nuclear program would add to regional uncertainties — possibly renewing Israeli threats to strike at Iran's nuclear facilities — there was no immediate indication that the timing of Iran's note to the I.A.E.A. was related to the events in Syria.

International negotiations on the nuclear program are stalemated by disagreement over the venue and date for the next encounter between Iranian negotiators and outside powers.

News of the Iranian note emerged days after Iran said it lofted a monkey into space as a prelude to human spaceflight in several years time.

While American analysts said the missile technology used in the experiment had little apparent military relevance, James E. Oberg, a former NASA engineer and author of a dozen books on human spaceflight, said Iran's civil space advances also had propaganda value, since the peaceful flights could take global attention off the nation's military feats and ambitions.

"Like the North Koreans, they get to present their program as peaceful when lots of it has to do with weapons development," he said.

The Vienna-based diplomat said the I.A.E.A. director general, Yukiya Amano, had circulated a note to the organization on Wednesday saying Iranian officials had informed the watchdog on Jan. 24 that "centrifuge machines type IR2m will be used" in a part of the Natanz enrichment plant.

The I.A.E.A. replied on Jan. 29, seeking technical information about the plan, Mr. Amano's note said.

Currently, Iran uses less reliable IR1 models developed in the 1970s, but has been reported for several years to be trying to enhance its enrichment capability with newer centrifuges developed domestically from technology initially acquired from Pakistan.

Last week, Israel's departing defense minister, Ehud Barak, said that the Pentagon had prepared sophisticated blueprints for a surgical operation to set back Iran's nuclear program should the United States decide to attack — a statement that was a possible indication that Israel might have shelved any plans for a unilateral strike, at least for now.

Iran's nuclear program also faces a threat of sabotage since a computer worm known as Stuxnet was used to attack its centrifuges more than two years ago. American intelligence officials believe the attack caused many of the machines to spin out of control and self-destruct, slowing the Iranian program's progress.

The Natanz plant, southeast of Tehran, is used to enrich uranium to less than four percent. But a newer uranium enrichment plant, known as Fordo, near the holy city of Qum, has raised Western concerns because it is buried deep underground, making it more impervious to scrutiny or attack.

The Fordo plant takes uranium fuel that has been enriched to 4 percent purity at Natanz and further enriches it to 20 percent purity, a level that can make fuel for a research reactor in Tehran. But it is far easier to make bomb-grade fuel from the 20-percenty concentration than from 4 percent, which has been one source of Western concern about Iran's ultimate intentions.

The Associated Press quoted a diplomat in Vienna as saying Iran had told the I.A.E.A. it planned to install over 3,000 of the IR2m centrifuges in addition to thousands of the older models at Natanz.