Hundreds of powerful US “bunker-buster” bombs are being shipped from California to the British island of Diego Garcia in the Indian Ocean in preparation for a possible attack on Iran.

The Sunday Herald can reveal that the US government signed a contract in January to transport 10 ammunition containers to the island. According to a cargo manifest from the US navy, this included 387 “Blu” bombs used for blasting hardened or underground structures.

Experts say that they are being put in place for an assault on Iran’s controversial nuclear facilities. There has long been speculation that the US military is preparing for such an attack, should diplomacy fail to persuade Iran not to make nuclear weapons.

Although Diego Garcia is part of the British Indian Ocean Territory, it is used by the US as a military base under an agreement made in 1971. The agreement led to 2,000 native islanders being forcibly evicted to the Seychelles and Mauritius.

The Sunday Herald reported in 2007 that stealth bomber hangers on the island were being equipped to take bunker-buster bombs.

"They are gearing up totally for the destruction of Iran" Dan Plesch, director, Centre for International Studies and Diplomacy, University of London

Contract details for the shipment to Diego Garcia were posted on an international tenders’ website by the US navy.

A shipping company based in Florida, Superior Maritime Services, will be paid $699,500 to carry many thousands of military items from Concord, California, to Diego Garcia.

Crucially, the cargo includes 195 smart, guided, Blu-110 bombs and 192 massive 2000lb Blu-117 bombs.

“They are gearing up totally for the destruction of Iran,” said Dan Plesch, director of the Centre for International Studies and Diplomacy at the University of London, co-author of a recent study on US preparations for an attack on Iran. “US bombers are ready today to destroy 10,000 targets in Iran in a few hours,” he added.

The preparations were being made by the US military, but it would be up to President Obama to make the final decision. He may decide that it would be better for the US to act instead of Israel, Plesch argued.

“The US is not publicising the scale of these preparations to deter Iran, tending to make confrontation more likely,” he added. “The US ... is using its forces as part of an overall strategy of shaping Iran’s actions.”

According to Ian Davis, director of the new independent thinktank, Nato Watch, the shipment to Diego Garcia is a major concern. “We would urge the US to clarify its intentions for these weapons, and the Foreign Office to clarify its attitude to the use of Diego Garcia for an attack on Iran,” he said.

For Alan Mackinnon, chair of Scottish CND, the revelation was “extremely worrying”. He stated: “It is clear that the US government continues to beat the drums of war over Iran, most recently in the statements of Secretary of State, Hillary Clinton.

“It is depressingly similar to the rhetoric we heard prior to the war in Iraq in 2003.”

The British Ministry of Defence has said in the past that the US government would need permission to use Diego Garcia for offensive action. It has already been used for strikes against Iraq during the 1991 and 2003 Gulf wars.

About 50 British military staff are stationed on the island, with more than 3,200 US personnel. Part of the Chagos Archipelago, it lies about 1,000 miles from the southern coasts of India and Sri Lanka, well placed for missions to Iran.