The US is secretly upgrading special stealth bomber hangars on the British island protectorate of Diego Garcia in the Indian Ocean in preparation for strikes on Iran's nuclear facilities, according to military sources.
The improvement of the B1 Spirit jet infrastructure coincides with an "urgent operational need" request for £44m to fit racks to the long-range aircraft.
That would allow them to carry experimental 15-ton Massive Ordnance Penetrator (MOP) bombs designed to smash underground bunkers buried as much as 200ft beneath the surface through reinforced concrete.
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One MOP - known as Big Blu - has already been tested successfully at the US Air Force proving ground at White Sands in New Mexico. Tenders have now gone out for a production model to be ready for use in the next nine months.
The "static tunnel lethality test" on March 14 completely destroyed a mock-up of the kind of underground facility used to house Iran's nuclear centrifuge arrays at Natanz, about 150 miles from the capital, Tehran.
Although intelligence estimates vary as to when Iran will achieve the know-how for a bomb, the French government recently received a memo from the International Atomic Energy Agency stating that Iran will be ready to run almost 3000 centrifuges in 18 cascades by the end of this month. That is in defiance of a UN ban on uranium enrichment and would be enough to produce a nuclear weapon within a year.
Diego Garcia, part of Britain's Indian Ocean Territory, has several current missions. US Air Force bombers and Awacs surveillance planes operate from its 12,000ft runway and the USAF Space Command has built a satellite tracking station and communications facility.
The Ministry of Defence says the US government would need Britain's permission to use the island for offensive action. It has already been used for strategic strike missions during the 1991 and 2003 Gulf wars against Iraq.
The UK "sovereign territory" has a garrison of 50 British and 3200 US military personnel.
The atoll, the largest in the Chagos Archipelago chain, lies about 1000 miles from the southern coasts of India and Sri Lanka. It is ideally placed for strategic missions in the Middle East.
The US Department of Defence request for special bomb racks was hidden in a £95bn request to the US Congress last week for extra emergency funding for the wars in Iraq and Afghanistan.
The new Big Blu bomb is 20ft long, weighs 30,000lb and carries 6000lb of high explosives. It is designed to go deeper than even existing nuclear bunker-busting weapons.
The bomb is designed to be dropped from as great a height as possible to achieve maximum velocity and penetrating power, guided on to target by satellite and accurate to within a few feet.
Each B2 bomber would be able to carry only one weapon because of its weight. The B2s, normally based at Barksdale, Missouri, flew round-trip strikes against Baghdad in 2003, but would ideally be positioned closer to its targets for missions against Iran.
The Pentagon has drawn up contingency plans for a range of attacks on Iran. The likeliest is a five-day bombardment, aiming to disable nuclear facilities and all major airbases and radar facilities; the most devastating would involve air and cruise missile attacks on 1000 targets, including headquarters and barracks of the Iranian Republican Guard Corps, over more than a month.
The US branded the Revolutionary Guards a terrorist organisation last week in the latest round of diplomatic sanctions against Tehran.