The assassination of Iranian nuclear scientist Mostafa Ahmadi Roshan came less than 24 hours after Israel's military chief warned that the Tehran regime could face "unnatural" events during the critical year ahead, fuelling speculation that the hand of the fabled Israeli intelligence service the Mossad was behind the latest attack.

Benny Gantz, the Israeli Defence Forces chief of staff, told a parliamentary committee: "For Iran, 2012 is a critical year in combining the continuation of its nuclearisation, internal changes in the Iranian leadership, continuing and growing pressure from the international community and things which take place in an unnatural manner."

These "unnatural" events are said to amount to a covert war aimed at hampering the development of Iran's nuclear capacity that has seen a series of mysterious assassinations and explosions in Iran over the past two years.

After the latest explosion, caused by magnetic bombs attached to the side of Roshan's car by an assailant on a motorcycle, the Iranian regime was quick to blame Israel. "This terrorist act was carried out by agents of the Zionist regime, with the aim of stopping our scientists," the vice-president, Mohammad Reza Rahimi, told state television.

Le Figaro, the French daily, swiftly weighed in with a report claiming that Israeli intelligence had recruited Iranian dissidents from Iraqi Kurdistan to gather information on Iran's nuclear programme and target its experts.

The Israeli government made no comment about the killing or suggestions that its agents were involved. "It's not our policy to comment on this sort of speculation," said a government official.

But normally talkative analysts and former officials were also saying little. "We've all read the same spy novels," one said laconically.

Ilan Mizrahi, former head of the national security council and an ex-deputy chief of the Mossad, said: "I'm not talking about the Mossad, or something that somebody somewhere thinks it has done. The Mossad is supposed to be behind everything from the tsunami to sharks in the Red Sea."

But suggestions of Israeli involvement are unlikely to go away. Two years ago on Thursday, an Iranian nuclear scientist was killed by a bomb outside his house in Tehran, and since than there have been three further mysterious deaths, including Roshan's, plus a series of unexplained explosions.

Some believe this adds up to a secret war waged by Israel while it simultaneously presses the international community for tougher sanctions and prepares for a possible military strike.

The German news organisation Spiegel published a report last August quoting unnamed Israeli security sources saying the Jewish state was waging a covert war on Iran, suggesting the policy was being driven by the Mossad's new chief, Tamir Pardo, who took up his post on 1 January 2011.

Two months ago, the Mossad was speculatively linked to a blast at Bid Ganeh, an Iranian military facility, in which 17 people were killed. Time quoted a western intelligence source claiming it was the work of the Mossad, and saying: "There are more bullets in the magazine."

Also in November, the Iranian authorities said they had arrested 12 people they claimed were CIA agents working in liaison with the Mossad. Parviz Sorouri, a member of the Iranian government's foreign policy and national security committee, said: "The US and Zionist regime's espionage apparatuses were trying to damage Iran both from inside and outside with a heavy blow, using regional intelligence services. Fortunately, with swift reaction by the Iranian intelligence department, the actions failed to bear fruit."

However, as one Israeli official pointed out, disinformation obscured the true picture. "There is so much opacity, it's not possible to connect the dots," the official said. "I don't think we are the only ones interested in troubling Iran, and sometimes things are attributed to the usual suspects when it isn't necessarily so."

The Mossad has a long track record of meticulously planned covert operations and targeted assassinations outside Israel. It was suspected of killing Mahmoud al-Mabhouh, a Hamas commander, in a Dubai hotel two years ago during an operation in which its agents used passports stolen and cloned from British, Irish and other citizens.

As well as targeting pro-Palestinian militants, the Mossad was also allegedly responsible for the death in 2008 of Muhammad Suleiman, reported to be the head of Syria's nuclear programme, who was shot dead from a boat while on a beach.

The Israeli government and security establishment never admits to its covert operations.