Tehran: Iran announced on Tuesday that it has put the elite Revolutionary Guards in charge of defending the country's territorial Arabian Gulf waters in what appeared to be a hardening of its stance in the vital oil route.

US commanders in the Gulf have in the past said they find Guards ships more confrontational than the regular Iranian navy, which until the new order was responsible for Iranian defences in the Gulf.

Iran has warned repeatedly that it will close the narrow Hormuz Strait at the mouth of the Gulf if the US or Israel attacks it amid tensions over Iran's nuclear programme.

Around 40 per cent of the world's oil passes through Hormuz. Last winter, Iranian and US ships patrolling the Gulf had a series of small confrontations in Hormuz that the Americans blamed on provocations by Guards ships.

General Yahya Rahim Safavi, the top military adviser of Iran''s supreme leader Ayatollah Ali Khamenei, announced the change on the state news agency IRNA.

He said 'responsibility to defend the Gulf' has been delegated to the Guards' navy, while the regular navy would operate in the Gulf of Oman, outside the Gulf and in the landlocked Caspian Sea.

Safavi, who was the head of the Guards until earlier this year, added a warning that all vessels in the Gulf are within the range of Iranian missiles.

"The Guards' missiles are able to cover the whole width of the Arabian Gulf. No warship can pass through the waterway without being in our range," he said. "Our armed forces, possessed with defensive weapons including missiles, air, sea and torpedoes, are able to control the strait of Hormuz."

A spokesman for the US Navy in Bahrain said the Iranian announcement will not significantly affect the Fifth Fleet's patrols of the Arabian Gulf, including the strategic waterway.

Navy Lieutenant Nathan Christensen said their task is to keep the Strait of Hormuz open to ensure "free flow of trade and commerce in and out of the region".

"We are not interested in a confrontation in the Gulf," said Christensen, speaking from the Fifth Fleet's Mideast headquarters in Manama. He added that the Navy expects "responsible and professional maritime behaviour" of all vessels in the Gulf, including those of the Iranian Guards.

The US Navy normally keeps some three dozen warships and auxiliary vessels in the area. It currently has one aircraft carrier operating in the Gulf.

Safavi, the Guards' former chief, also repeated warnings that Iran would retaliate against US bases in the Gulf if Israel launches a strike against Iran.

"The Zionist regime doesn't have the necessary political, economic and social capabilities to start a big war," he said. If Israel attacks, US forces in the region "will be put in serious danger. Definitely, the Americans don't want to get involved in a fourth front after Afghanistan, Iraq and Georgia."

Israel, which is believed to possess a nuclear arsenal, has warned it would attack Iran to prevent the country from acquiring nuclear weapons. The US says it seeking a diplomatic solution, but has not ruled out military action. Iran denies it intends to develop nuclear weapons.

The Guards corps, which has land, navy and air components, is considered better-equipped than the normal military and more ideologically fervent, tasked with protecting Iran's government.

The Guards' vessels stepped up patrols in Hormuz last year during a period when the US had increased its naval strength in the Gulf, making a show of strength over Iran's defiance of UN resolutions on its nuclear programme. The Guards say their navy vessels ask ships to identify themselves before entering the Gulf.

Despite last year's frictions, US Navy and Iranian officials say their vessels are in frequent radio contact to avoid incidents in the Gulf waters.

The Guards corps, which has land, navy and air components, is considered better-equipped than the militaryand more ideologically fervent.