The Dimona complex in the Negev desert is famous as the heavily guarded heart of Israel’s never-acknowledged nuclear arms program, where neat rows of factories make atomic fuel for the arsenal.

Over the past two years, according to intelligence and military experts familiar with its operations, Dimona has taken on a new, equally secret role — as a critical testing ground in a joint American and Israeli effort to undermine Iran’s efforts to make a bomb of its own.

Behind Dimona’s barbed wire, the experts say, Israel has spun nuclear centrifuges virtually identical to Iran’s at Natanz, where Iranian scientists are struggling to enrich uranium. They say Dimona tested the effectiveness of the Stuxnet computer worm, a destructive program that appears to have wiped out roughly a fifth of Iran’s nuclear centrifuges and helped delay, though not destroy, Tehran’s ability to make its first nuclear arms.

“To check out the worm, you have to know the machines,” said an American expert on nuclear intelligence. “The reason the worm has been effective is that the Israelis tried it out.”

Though American and Israeli officials refuse to talk publicly about what goes on at Dimona, the operations there, as well as related efforts in the United States, are among the newest and strongest clues suggesting that the virus was designed as an American-Israeli project to sabotage the Iranian program.

In recent days, the retiring chief of Israel’s Mossad intelligence agency, Meir Dagan, and Secretary of State Hillary Rodham Clinton separately announced that they believed Iran’s efforts had been set back by several years. Mrs. Clinton cited American-led sanctions, which have hurt Iran’s ability to buy components and do business around the world.

The gruff Mr. Dagan, whose organization has been accused by Iran of being behind the deaths of several Iranian scientists, told the Israeli Knesset in recent days that Iran had run into technological difficulties that could delay a bomb until 2015. That represented a sharp reversal from Israel’s long-held argument that Iran was on the cusp of success.

The biggest single factor in putting time on the nuclear clock appears to be Stuxnet, the most sophisticated cyberweapon ever deployed.

In interviews over the past three months in the United States and Europe, experts who have picked apart the computer worm describe it as far more complex — and ingenious — than anything they had imagined when it began circulating around the world, unexplained, in mid-2009.

Many mysteries remain, chief among them, exactly who constructed a computer worm that appears to have several authors on several continents. But the digital trail is littered with intriguing bits of evidence.

In early 2008 the German company Siemens cooperated with one of the United States’ premier national laboratories, in Idaho, to identify the vulnerabilities of computer controllers that the company sells to operate industrial machinery around the world — and that American intelligence agencies have identified as key equipment in Iran’s enrichment facilities.

Seimens says that program was part of routine efforts to secure its products against cyberattacks. Nonetheless, it gave the Idaho National Laboratory — which is part of the Energy Department, responsible for America’s nuclear arms — the chance to identify well-hidden holes in the Siemens systems that were exploited the next year by Stuxnet.

The worm itself now appears to have included two major components. One was designed to send Iran’s nuclear centrifuges spinning wildly out of control. Another seems right out of the movies: The computer program also secretly recorded what normal operations at the nuclear plant looked like, then played those readings back to plant operators, like a pre-recorded security tape in a bank heist, so that it would appear that everything was operating normally while the centrifuges were actually tearing themselves apart.

The attacks were not fully successful: Some parts of Iran’s operations ground to a halt, while others survived, according to the reports of international nuclear inspectors. Nor is it clear the attacks are over: Some experts who have examined the code believe it contains the seeds for yet more versions and assaults.

“It’s like a playbook,” said Ralph Langner, an independent computer security expert in Hamburg, Germany, who was among the first to decode Stuxnet. “Anyone who looks at it carefully can build something like it.” Mr. Langner is among the experts who expressed fear that the attack had legitimized a new form of industrial warfare, one to which the United States is also highly vulnerable.

Officially, neither American nor Israeli officials will even utter the name of the malicious computer program, much less describe any role in designing it.

But Israeli officials grin widely when asked about its effects. Mr. Obama’s chief strategist for combating weapons of mass destruction, Gary Samore, sidestepped a Stuxnet question at a recent conference about Iran, but added with a smile: “I’m glad to hear they are having troubles with their centrifuge machines, and the U.S. and its allies are doing everything we can to make it more complicated.”

In recent days, American officials who spoke on the condition of anonymity have said in interviews that they believe Iran’s setbacks have been underreported. That may explain why Mrs. Clinton provided her public assessment while traveling in the Middle East last week.

By the accounts of a number of computer scientists, nuclear enrichment experts and former officials, the covert race to create Stuxnet was a joint project between the Americans and the Israelis, with some help, knowing or unknowing, from the Germans and the British.

The project’s political origins can be found in the last months of the Bush administration. In January 2009, The New York Times reported that Mr. Bush authorized a covert program to undermine the electrical and computer systems around Natanz, Iran’s major enrichment center. President Obama, first briefed on the program even before taking office, sped it up, according to officials familiar with the administration’s Iran strategy. So did the Israelis, other officials said. Israel has long been seeking a way to cripple Iran’s capability without triggering the opprobrium, or the war, that might follow an overt military strike of the kind they conducted against nuclear facilities in Iraq in 1981 and Syria in 2007.

Two years ago, when Israel still thought its only solution was a military one and approached Mr. Bush for the bunker-busting bombs and other equipment it believed it would need for an air attack, its officials told the White House that such a strike would set back Iran’s programs by roughly three years. Its request was turned down.

Now, Mr. Dagan’s statement suggests that Israel believes it has gained at least that much time, without mounting an attack. So does the Obama administration.

For years, Washington’s approach to Tehran’s program has been one of attempting “to put time on the clock,” a senior administration official said, even while refusing to discuss Stuxnet. “And now, we have a bit more.”

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