Divers leap from a helicopter into the Gulf's choppy waters to destroy a mine, while a US marine fires at a floating target, as forces from 41 countries gathered for naval manoeuvres facing Iran's shores.

The mine-sweeping exercise is being held close to the Strait of Hormuz, the strategic mouth of the Gulf that Iran has repeatedly threatened to block if attacked over its disputed nuclear programme.

The US Navy's 5th fleet invited a group of journalists to attend part of the 25-day exercises which conclude on May 30.

This operation is "purely defensive" and targets no particular country, said Vice-Admiral John Miller, commander of the Bahrain-based 5th Fleet.

But Tehran warned against "provocations" last week as the US-led forces prepared for the exercise in the Gulf.

"All those present must be careful not to carry out provocative actions," said Iranian foreign ministry spokesman Ramin Mehmanparast.

A total of 35 ships are in the Gulf for the International Mine Countermeasures Exercise (IMCMEX), including the USS Ponce, an amphibious transport that has been modified to act as a floating base for mine-sweeping operations.

Also present are the USS Ardent, a mine countermeasures ship, Britain's RFA Cardigan Bay landing ship, 18 Unmanned Underwater Vehicles (UUVs) and 6,500 naval personnel.

During the exercise, divers scaled down ropes from Seahawk helicopters near floating mines, stuck explosives to the devices and detonated them remotely.

For underwater mines, the UUVs, remotely controlled from the "war room" aboard the USS Ponce, were sent to place an explosive device next to the mine to take it out.

IMCMEX "is about increasing our capabilities working with our international partners... A defensive exercise in nature," said Lieutenant Commander Peter Abbott aboard USS Ardent.

He said that in addition to mine countermeasures, the exercise this year included maritime infrastructure protection, as well as protection for oil platforms in the Gulf.

Alongside the US navy, forces from Britain, France and Germany and several Arab countries are taking part.

"For this exercise, the French presence is more important than last year," said Captain Frederic Benon, adding that the French navy sent a ship and divers for the manoeuvres.

Abbott stressed the need for the exercise by pointing out that waterways should be protected from attempts to block maritime traffic. The task is particularly significant in the case of the Strait of Hormuz, through which a third of global oil shipped by sea passes.

"International Waters need to remain open," Abbott said.

Iran's Revolutionary Guard is tasked with blocking the Strait of Hormuz in the event of a conflict, but has never been called upon to do it.

But in an apparent response to the exercise, Iran launched its own mine-sweeping exercise east of the Strait of Hormuz, in the Gulf of Oman, state Fars news agency said last week, adding that Iranian naval forces unveiled a "modern anti-mine" system.