"They will not close it... They will not be allowed to close it," Vice-Admiral Kevin J. Cosgriff said at a press conference in Bahrain, where the Fifth Fleet is based.

His remarks followed comments by the chief of Iran's elite Revolutionary Guards who issued a new warning last week against any attack against the country over its controversial nuclear drive.

"It is natural that when a country is attacked it uses all of its capabilities against the enemy, and definitely our control of the Persian Gulf and the Strait of Hormuz would be one of our actions," Jafari said.

The strait between Iran and Oman is a vital conduit for energy supplies, with as much as 40 percent of the world's crude oil from Gulf oil producers passing through the waterway.

There have been several confrontations between Iranian and US vessels in the Gulf this year.

Iran has defied UN sanctions and international demands by pressing ahead with its disputed uranium enrichment programme, which both Washington and Israel fear could be used to build a nuclear weapon.

Tehran, a leading member of the oil cartel OPEC, denies wanting the bomb, and says its nuclear ambitions extend only to generating electricity for a growing population.

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