Iran is planning to begin uranium enrichment soon at a new subterranean facility, bringing forward the moment when Israel and the United States will have to decide whether to launch a military strike.

Meanwhile, the clandestine campaign against the nuclear programme continues. This week another Iranian nuclear scientist was killed in Tehran.

The existence of a second uranium enrichment facility under construction, at Fordo, near the city of Kom, was revealed in September 2009.

Though Iran has promised to allow the International Atomic Energy Agency inspectors to visit the underground site, intelligence services in the West and Israel believe that its main purpose is to allow Iran to "break-out" and swiftly enrich uranium to weapons-grade purity, once the decision to go ahead is taken in Tehran.

Fereydoun Abbasi, the director of Iran's atomic energy organisation, announced on Saturday that the Fordo facility will begin operating soon. He said that the new facility would produce uranium of twenty per cent purity.

While civilian nuclear reactors need uranium of only five per cent purity to operate, it takes only a few months to produce weapons-grade uranium of 93 per cent purity from the starting point of higher level 20 per cent purity uranium.

The main concern is that Iran will soon transfer the necessary components for enrichment to the new underground plant, hindering a possible attack by the United States or Israel.

US Secretary of Defence Leon Panetta said on Sunday that Iran was laying the groundwork to build a nuclear bomb but had not yet decided actually to build one.

On Wednesday morning, two Iranians were killed when a car exploded in Tehran.

One was Mostafa Ahmadi Roshan, a chemistry professor and a director at the Natanz uranium enrichment facility. This was the fifth such explosion over the last two years in Iran, in which four nuclear scientists have been killed. No -one has claimed responsibility for the bombing, although Iran has accused Israel, the US and Britain over the attacks.

IDF Chief of Staff Lieutenant General Benny Gantz told a Knesset committee this week that 2012 would be a "critical year" for Iran.

The Institute for National Security Studies at Tel Aviv University reported that "Iran is closer than ever to the junction where the Iranian leadership has to decide whether to remain in the relatively comfortable position, on the threshold of nuclear capacity, or to "break-out" for a bomb."

The INSS said it was in Iran's interest to postpone to a later stage the decision whether to cross the threshold. But it believed that " a series of regional and international developments could cause it to decide to speed up nuclear development and break out to a nuclear weapon."