The US Navy said it was carrying out an exercise in the Gulf, days after vowing that Iran will not be allowed to block the waterway which carries crude from the world's largest oil-exporting region.

'The aim of 'Exercise Stake Net' is to practice the tactics and procedures of protecting maritime infrastructure such as gas and oil installations,' Commodore Peter Hudson said in a US Fifth Fleet statement.

The head of Iran's Revolutionary Guards said in remarks published late last month that Tehran would impose controls on shipping in the Gulf and the strategic Strait of Hormuz if it was attacked.

Speculation about a possible attack on Iran because of its nuclear program has risen since a report last month said Israel had practiced such a strike.

Vice-Admiral Kevin Cosgriff, the commander of the US Navy's Fifth Fleet, said last week the United States would not allow Iran to block the Gulf.

Fear of an escalation in the standoff between the West and Iran, the world's fourth largest oil producer, has helped propel oil prices over $140 a barrel.

Two US vessels were taking part in the exercise alongside a British warship and one from Bahrain, a Gulf Arab ally which hosts the Fifth Fleet.

'Stake Net seeks to help ensure a lawful maritime order as well as improve relationships between regional partners,' the fleet's statement said.

Western powers say they fear Iran is seeking to develop nuclear weapons under cover of a civilian nuclear program. Tehran says the work aims to generate electricity.

A cargo ship hired by the US military fired warning shots at two unidentified boats which approached it in the Gulf in April.

In January, the United States said five small Iranian speedboats aggressively approached three US Navy ships in the Strait of Hormuz and a radio message was transmitted, warning they could explode.

Iran said its boats were simply trying to identify the U.S. vessels and military experts have since said the warning may have come from an independent radio operator.-Reuters