WASHINGTON  Israel and America are intensifying a clandestine war against Iran that has run hot and cold since the Islamic Revolution in 1979 but has grown more urgent as Iran races to obtain an atomic bomb.

That is a central claim in a new book, "The Secret War with Iran," by an Israeli journalist, Ronen Bergman, who also details a series of mishaps during the past 2 1/2 years that have likely delayed Iran's efforts to go nuclear.

While President Bush and other Western leaders have warned of the seriousness of the threat that Iran may obtain a nuclear weapon, little reporting has surfaced in the West on the efforts in the shadows to stop the Iranians. Mr. Bergman himself has had to skate a close line in this area, in part because of military censorship in Israel, where some of his reporting has been withheld from publication pending rulings from the Israeli Supreme Court.

Nonetheless, the Israeli journalist compiles a picture that suggests that the West has had some successes in the war to stop the Iranian bomb. Mr. Bergman reports, for example, that in January 2007 Iran determined that some of its nuclear suppliers in Europe were fronts for Western intelligence services, specifically Britain's MI6.

And Mr. Bergman writes that between February 2006 and March 2007, at least three planes "belonging to the Revolutionary Guards crashed in Iran, while carrying personnel connected with the security of the nuclear project." Specialized pipes for centrifuges sold to Iran have been modified, he writes, and specialized computers sold to Iran for its nuclear laboratories contained viruses that sabotaged the code.

The secret efforts appear not to be limited to modifying equipment: On January 18, 2007, an Iranian expert on electromagnetics who worked in an Isfahan enrichment facility, Ardeshir Hosseinpour, died in his apartment, Mr. Bergman writes.

The author quotes the deputy director of the Israeli Atomic Energy Commission, Eli Levite, as saying in a closed forum that operations against Iran "gained time for us" and have "doubtless caused significant delays in the project. The process has led to the revealing of large parts of the program in the areas of sources of supply, of the infrastructure, and of the goals, which were not known or were known at a different resolution."

While Israel's Mossad and military intelligence have targeted Iranian terrorists almost since the 1979 revolution, the Jewish state was relatively slow to pick up on the full extent of Iran's nuclear program. Mr. Bergman reports that Israel first learned of the nuclear facility in Natanz in 1996, a full six years before the facility was first disclosed to the public, but several years after the Iranians began their initial work there. Two Israeli operatives, posing as tourists, arrived at the site and took soil samples, which they brought back to Israel in their shoes and which showed some radiation.

Mr. Bergman also details a success for the CIA in the shadow war against Iran, when General Ali Reza Askari defected to the American side in February 2007. Mr. Bergman reports that General Askari was closer to the reformist President Khatami and felt threatened by his old rival in intelligence when President Ahmadinejad came to power in 2005.

General Askari, for example, warned Mr. Khatami after the attacks of September 11, 2001, that Iran's Revolutionary Guards had given shelter to key Al Qaeda operatives fleeing American troops in Afghanistan. He said in his debriefings, according to Mr. Bergman, that Iran had entered into joint nuclear projects with both Syria and North Korea.

The defector also claimed that Iran erected a secret enrichment facility near the known centrifuge area in Natanz.

Mr. Bergman finally comes close to saying outright that Israel was responsible for the assassination in February of a master Hezbollah terrorist, Imad Mugniyah. He writes: "Although Israel has denied responsibility for the assassination, the Mugniyah hit was exactly the kind of thing needed to restore the country's faith in, and more importantly the enemy's fear of, Israel's intelligence services."

Mr. Bergman then quotes an Israeli intelligence official, who recalls the exact model of the vehicle Mugniyah was driving when he was attacked. "Pity about that new Pijero," he said.