The United States has successfully tested missiles designed to destroy Iran's underground nuclear laboratories.

In information passed to Israel, the US government said that tests last year of the GBU-57B had gone well.

The massive ordnance penetrator, as the weapon is called, has cost $500 million to develop, and can be dropped from B-2 stealth bombers.

The 13-ton weapon travels at twice the speed of sound and can puncture up to 200ft of concrete. The report states that it is accurate to within 16 feet.

Experts believe that the US communicated the test results in order to convince Israel of their resolve to prevent Iran developing nuclear weapons, and to dissuade Israel from launching a pre-emptive strike.

Michael Gilmore, Pentagon director of operational testing, told Congress in January that the weapon was ready to go into service, after permission was given to develop it three years ago.

The Iranian government is believed to have a facility for developing nuclear weapons near the city of Natanz, which is covered in 80 feet of concrete and rock, stretches over seven acres and contains laboratories, offices and centrifuge halls.

An Israeli source told the Times: "Although we are all preoccupied with Syria, the clock in Natanz is ticking and the endgame is approaching."

Speaking of the weapon, the source said, "It's very impressive, but will they have the guts to get involved in another Middle Eastern war?"

However, perhaps in anticipation of a strike, the Iranians have moved part of their enrichment programme to Fordow, where they have constructed a bunker 300 feet below a mountain.

Last week, Iran's eight presidential candidates argued over the country's nuclear strategy in televised debates, with centrist Hasan Rowhani suggesting a more conciliatory stance at the negotiating table and conservative Saeed Jalili accusing Rowhani of appeasing the West.

Israel's Prime Minister Binyamin Netanyahu has stated that that the presidential elections in Iran will have no impact on the country's nuclear weapons programme or on arming Syria, Hezbollah, Hamas and Islamic Jihad.