These days, more than ever, news from Iran is surrounded in mystery. Whether it's the assassination of nuclear scientists, explosions at military bases, the spread of a computer worm or even the downing (or crash) of a US spy drone, it is difficult to establish with certainty what is really happening.

Mostafa Ahmadi Roshan, a key figure at the Natanz uranium enrichment plant in central Iran, died in the latest incident, on Wednesday morning.

According to initial reports, two attackers on a motorcycle attached magnetic bombs to Roshan's car, killing him and injuring others. Two other nuclear scientists, Masoud Ali Mohammadi and Majid Shahriari, were killed in similar attacks, one in January 2010, the other in November 2010. Fereydoon Abbasi Davani, Iran's current atomic chief, survived an assassination on the day his colleague, Shahriari, was targeted.

In July last year, Darioush Rezaeinejad, an Iranian academic, was shot dead by gunmen riding on motorcycles. Iran denied that he was involved in nuclear work and one theory is that he was killed in mistake for a scientist with a similar name.

Last November, an explosion was heard in the city of Isfahan, close to sensitive nuclear facilities. Another explosion at a military base killed the architect of the Islamic regime's missile programme along with 16 more of its elite revolutionary guards. A similar blast last year hit a missile base in Khorramabad, Iran's nearest point to Israel.

Stuxnet, a computer worm believed to have been designed to sabotage Iran's enrichment of uranium hit many of the country's centrifuges last year.

Then there was the mysterious affair of the US drone reported missing in Afghanistan, which later turned up in Iran.

It's difficult to view all these incidents as unrelated. Taken as a whole, they suggest that opponents of the Iranian regime have launched a covert campaign aimed at disrupting Iran's nuclear and missile programmes – possibly as an alternative to the more costly option of overt war.

No one has claimed responsibility. Israel is seen as a natural suspect, not only because it has refused to deny involvement but also because of its history of covert operations. The Mossad's kidnapping of the nuclear whistleblower Mordechai Vanunu, and the systematic killings of people involved in the Black September massacre are just two examples.

Whether or not Israel is behind this campaign in Iran is a mystery. An Israeli military chief was quoted this week as saying cryptically that Iran should expect more "unnatural" events in 2012.

Iran, on the other hand, has pointed a finger at both Israel and the US. Unlike Israel, the US has denied involvement in the assassination of scientists.

Demonstrations at the British embassy in Tehran last November, where protesters carried pictures of the assassinated Shahriari, showed that Iran sees Britain in the loop too.

Iran is right to be suspicious about Britain. Sir John Sawers, the head of MI6, was once caught on the record endorsing covert actions against Iran. "We need intelligence-led operations to make it more difficult for countries like Iran to develop nuclear weapons," he said in a speech in 2010. The role of the Secret Intelligence Service, he added, "is to find out what these states are doing … and identify ways to slow down their access to vital materials and technology".

Sawers's views are echoed by both US and Israeli officials. "We are not happy to see the Iranians move ahead on this [nuclear programme], so any delay, be it divine intervention or otherwise, is welcome," Israel's defence minister, Ehud Barak said recently. In October, a senior US general, Jack Keane, reacted to reports of an Iranian plot to kill the Saudi ambassador in Washington by saying: "Why don't we kill them [the revolutionary guards]? … We've got to put our hand around their throat right now."

Such comments do not prove that any of these countries is actually engaged in a covert war against Iran. But no matter who is responsible for the extrajudicial killings and apparent sabotage, one thing should be considered above all: these are illegal actions under international law.

Whether it's an individual simply murdering people or a foreign state inflicting injuries upon the nationals of another state and violating the territorial sovereignty of the Islamic republic, international laws and human rights conventions prohibit such activities.

Supporters of covert war against Iran see it as an alternative to aerial bombing raids or full-scale war. They believe it's a better approach (even though it is illegal) since there are fewer civilian casualties and public confrontation with supporters of Iran, such as Russia and China, can be avoided.

But illegal action will only ruin any chance of dialogue with Tehran. It will encourage Iran to be less prudent and become more radical about its nuclear activities and – most importantly – will encourage Iran to react in a similar fashion with its own covert operations. The covert war against Iran, if not stopped, could escalate out of control.

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