Call it Plan B for dealing with Iran, which recently revealed a long-suspected nuclear site deep inside a mountain near the holy city of Qom. The 15-tonne behemoth, called the “massive ordinance penetrator,” or MOP, will be the largest non-nuclear bomb in the US arsenal and carry 2,400 kg of explosives. The bomb is about 10 times more powerful than the weapon it is designed to replace.

The Pentagon has awarded a nearly US $52 million contract to speed up placement of the bomb aboard the B-2 Stealth bomber, and officials say the bomb could be fielded as soon as next summer.

No specific target

Pentagon officials acknowledge that the new bomb is intended to blow up fortified sites like those used by Iran and North Korea for their nuclear programs, but they deny there is a specific target in mind.

“I don’t think anybody can divine potential targets,” Pentagon press secretary Geoff Morrell said. “This is just a capability that we think is necessary given the world we live in.”

The Obama administration has struggled to counter suspicions lingering from George W Bush’s presidency that the US is either planning to bomb Iranian nuclear facilities itself or would look the other way if Israel did the same.

Defence Secretary Robert Gates recently said a strike against Iran’s nuclear facilities would probably only buy time. Joint Chiefs of Staff Chairman Admiral Mike Mullen has called a strike an option he doesn’t want to use.

The new US bomb would be the culmination of planning begun in the Bush years. The Obama administration’s plans to bring the bomb on line more quickly indicate that the weapon is still part of the long-range backup plan.

“Without going into any intelligence, there are countries that have used technology to go further underground and to take those facilities and make them hardened,” Pentagon spokesman Bryan Whitman said. “This is not a new phenomenon, but it is a growing one.”

The MOP could, in theory, take out bunkers such as those Saddam Hussein had begun to construct for weapons programmes in Iraq, or flatten the kind of cave and tunnel networks that allowed al-Qaida leader Osama bin Laden to escape US assault in Tora Bora, Afghanistan, shortly after the US invasion in 2001.

The precision-guided bomb is designed to drill through earth and almost any underground encasement to reach weapons depots, labs or hideouts.