No one has claimed responsibility for the death of Mostafa Ahmadi Roshan, the deputy director of the uranium enrichment facility in Natanz, but senior officials in Tehran were quick to point the finger at "the mercenaries of the Zionist regime".

Israeli officials would not comment on the incident, which came a day after the chief of the Israeli Defence Forces, Lieutenant-General Benny Gantz, told a private parliamentary committee meeting Iran should expect "continuing and growing pressure from the international community and things which take place in an unnatural manner".

The 32-year-old nuclear scientist died on a crowded main street in Tehran when a man riding on the back of a motorbike attached a magnetic bomb to the side of his car, which was trapped in peak hour traffic.

His death is part of a pattern of assassinations, mysterious explosions and high-level computer sabotage in Iran over the past two years.

Israel's secretive intelligence agency, Mossad, had a long history of "taking aggressive measures to eliminate scientists connected to these projects", said Dr Ronen Bergman, the author of The Secret War With Iran and a senior policy analyst with Yedioth Ahronoth, a major daily newspaper in Israel.

Dr Bergman said the assassinations were aimed at achieving two main goals.

"The first is to neutralise, to take out, significant figures that are essential to the advancement of the proliferation project, and the second goal … is to spread fear amongst the scientists who are left behind."

Speaking at a conference at the Hebrew University in Israel earlier this week, Martin Indyk, a former US ambassador to Israel and director of foreign policy at the Brookings Institution in Washington, said the decision by President Barack Obama to sign into law the latest sanctions on Iran's central bank "moved the United States in a direction towards regime change".

"These sanctions have the potential, together with the ban on oil imposed by the Europeans, to seriously restrict the flow of revenues to the Iranian government.''

Mr Indyk rated the chances of the US using military force against Iran as "for the time being at least, zero".

Gary Sick, a specialist on Iran at Columbia University, said he believed that the covert campaign, combined with sanctions, would not persuade Iran to abandon its nuclear work.

''It's important to turn around and ask how the US would feel if our revenue was being cut off, our scientists were being killed and we were under cyber attack,'' Mr Sick said. ''Would we give in, or would we double down? I think we'd fight back, and Iran will, too.''

with The New York Times