Iran's nuclear program has fuelled an escalating war of words between Israel, which believes the Islamic state is well on its way to developing atomic weapons, and Iran, which maintains the nuclear technology it is developing is solely for civilian use.

The two countries have publicly threatened military action against each other, and Israeli Prime Minister Benjamin Netanyahu is expected to ask Prime Minister Stephen Harper to support an Israeli pre-emptive strike on Iran when he visits Ottawa on  March 2.

But there are growing indications that Israel and Iran have actually been fighting a covert war for years.

In the past two years, at least five Iranian nuclear scientists have been killed in car bombings. No links have been proven, but some intelligence experts think they were directed by Israel's spy agency, Mossad.

In mid-February 2012, Iran appeared to respond with a series of co-ordinated, though unsuccessful, bomb attacks on Israeli officials in India, Georgia and Thailand.

Signs of a 'cold war'

Experts say this secret war has been simmering since the mid-2000s.

"It's something we've been tracking since about 2006," says Scott Stewart, vice-president of tactical intelligence for Stratfor, a geopolitical analysis firm based in Austin, Tex.

There have been a number of incidents that suggest the two countries have been engaged in sabotage, but these have been impossible to verify given that neither side has claimed any responsibility.

According to Stewart, one of the first signs of a "cold war" between the two countries was the disappearance — sometime between December 2006 and February 2007 — of Ali Reza Asgari, a general in Iran's Revolutionary Guard, an elite wing of the military charged with defending the regime, and a former deputy defence minister. Asgari, who had reportedly been in Turkey to meet a European arms dealer, was thought to be a treasure trove of information for Western intelligence agencies and Mossad. There is debate about whether he defected or was kidnapped.

Stewart claims the Iranians retaliated by abducting Bob Levinson, a former FBI agent, in Iran in 2007 and later attacking an Israeli attaché for agriculture in Paris.

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Tensions between Iran and Israel intensified after multiple explosions that seemed to target Israeli officials occurred in Bangkok on Feb. 15. Here, Thai security guards inspect a car entering a parking lot in the Israeli Embassy a day after the attacks. (Nicolas Asfouri/AFP/Getty Images )

In January 2010, Massoud Ali Mohammadi was killed by a bomb blast outside his home in Tehran, the first of several Iranian nuclear scientists to die in mysterious circumstances. An Iranian man, who was thought to be a spy for Israel, confessed to the crime.

While Israel has never acknowledged complicity in such cloak-and-dagger activity, it has been quite vocal in denouncing Iran's nuclear aims.

For its part, the U.S. has publicly stuck by a 2007 U.S. intelligence assessment that said Iran had abandoned efforts to develop an atomic bomb in 2003. But in November 2011, the International Atomic Energy Association, which has been given access to some but not all of Iran's nuclear facilities over the years, said for the first time that it suspected Iran was conducting experiments whose only purpose was the manufacture of nuclear arms.

Stuxnet strikes blow

In 2010, the world first heard about a computer virus called Stuxnet. While it initially spread through Microsoft Windows, the worm specifically targeted Siemens software and equipment, which is the basis of Iran's uranium-enrichment infrastructure. Thought by computer security experts to have been created by Israeli or U.S. programmers, Stuxnet did significant damage to Iran's nuclear capabilities.

Spy games between Israel and Iran are 'something we've been tracking since about 2006.'— Scott Stewart, v-p of tactical intelligence for geopolitical analysis firm Stratfor

"Stuxnet was successful, in a short-term sense, by stalling Iran's program for a year and resulting in at least 1,000 gas centrifuges being effectively destroyed," says Paul Brannan, senior analyst at the Institute for Science and International Security in Washington, D.C.

Brannan makes a distinction between sabotage of the Stuxnet type, which he considers a valid way of undermining Iran's nuclear program, and harsher tactics such as assassinations, which he says "should stop."

"They're counterproductive," said Brannan. "They're targeting people who are scientists; they're not soldiers. And I don't think they're going to have any effect on Iran's program."

The consensus among military experts seems to be that with a couple of possible exceptions, the scientists who have been killed did not have unique knowledge of Iran's nuclear capabilities and were, ultimately, replaceable.

"The feeling is that this is more of an attempt to intimidate the scientific community than seriously slow down the work of the nuclear program," says Jeremy Binnie, Middle East and Africa editor for Jane's Defence Weekly.

MEK denies Mossad links

There are many theories about who might be physically carrying out such assassinations, including Iranian Jews and members of the Kurdish community, which seeks independence from Iran. The most widespread theory is that Mossad is using members of the Mojahedin-e Khalq (MEK), an exiled militant Iranian opposition group that was once supported by Saddam Hussein and is considered a terrorist organization by Canada and the U.S. (although the group has been lobbying the governments of both countries to change that designation).

"Supposedly, the Israelis are trying to train these guys and send them back into Iran," says Binnie.

The MEK has denied any association with Mossad and has described allegations that it is helping the agency carry out assassinations of Iranian scientists as "baseless" and "totally fabricated" by the Iranian regime.

Espionage between Iran and Israel predates the current conflict over nuclear capability. During the Israel-Lebanon war in the early 1980s, Iran is thought to have provided financing and operational support to Lebanese Shia militant groups Hezbollah and Islamic Jihad.

Over the next couple of decades, Iran was accused of using Hezbollah and Islamic Jihad to carry out attacks on Israeli nationals. One such incident was the 1992 attack on the Israeli embassy in Buenos Aires, which was thought to have been revenge for Israel's assassination of a high-ranking Islamic Jihad operative.

Events in the past half-decade suggest this tit-for-tat continues. After Iranian scientist Mostafa Ahmadi Roshan died in a car bombing on Jan. 11, 2012, there were several incidents that security experts saw as Iran-sponsored attempts at retaliation. Police in Bangkok apprehended a Lebanese national who had procured a large amount of fertilizer, which could be used to make explosives, while authorities in Azerbaijan reported that a group with ties to Iran had been targeting Israeli teachers in the capital, Baku.

Strange plots abound

Since this is espionage we're talking about, many of the attacks entail a series of proxies. Sometimes, the cast of characters can seem downright odd.

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Iranian President Mahmoud Ahmadinejad, seen here touring the Natanz uranium enrichment facility in 2007, maintains that his country's nuclear program is only used for power generation and other civilian purposes such as the production of medical isotopes. (Iranian President's Office/AP)

In October 2011, U.S. officials alleged they had uncovered a bizarre plot to assassinate the Saudi ambassador to the U.S. (Saudi Arabia is one of Iran's bitterest enemies.) According to the U.S., members of the Quds force, a special unit of Iran's army, had engaged a used-car salesman from San Antonio, Tex., to reach out to Mexico's Los Zetas drug cartel to do the deed. The Iranian leadership denied the charge and demanded an apology from the Obama administration.

"Honestly, when we heard [about the plot]

, we were kind of shaking our heads, thinking, this is really strange," says Stewart. "But then when we started looking at it, and we saw a previous case that was exposed by WikiLeaks of the Iranians using an unemployed house painter from Michigan to try to assassinate a dissident in Los Angeles as well as a dissident in London, it kind of becomes more believable."

To date, it seems as though Iran has incurred the greater losses in these clandestine operations. In addition to the deaths of five Iranian nuclear scientists, Israel is believed to be behind a November 2011 explosion at a military base near Tehran that killed 17 people, including Brig.-Gen. Hassan Moqqadam, the head of Iran's missile development program.

Stewart says the deployment of the sophisticated Stuxnet virus in 2010 was a watershed moment for cyber-warfare, and believes it also underscores the urgency of the mission to undermine Iran.

"The fact that they were willing to unveil this totally new form of warfare shows the importance they're placing on stopping the Iranian nuclear program," Stewart says.