Dubai (AFP) - A "flying object" which flew over a Japanese tanker before it was rocked by a blast in strategic Gulf waters last week could have been a reconnaissance drone, experts have told AFP.

The owner of the Kokuka Courageous said the tanker's Japanese and Filipino crew saw a "flying object", just before a blast that caused a fire on board the vessel, sparking a crisis between Washington and Iran.

"The crew members are saying that they were hit by a flying object. They saw it with their own eyes," Yutaka Katada, head of Kokuka Sangyo shipping company, said the day after the mysterious June 13 attack.

"We have received a report saying that something seems to have flew in, there was an explosion and it created a hole in the body of the ship," he told reporters in Tokyo.

Katada said the unidentified object flew over the methanol-laden tanker a first time -- and returned a second time three hours later, when the blast went off.

The Kokuka Courageous was hit around the same time as another tanker in the area, the Norwegian-owned Front Altair, which was hit by three explosions, according to the Norwegian Maritime Authority.

They were transiting through the strategic Strait of Hormuz towards the Indian Ocean.

The United States -- as well as Britain and Saudi Arabia -- have accused Iran of being behind the attacks but Tehran has denied any involvement.

The US navy says limpet mines, classic maritime weapons used during World War II, were placed above the water level on the hull of the Kokuka Courageous, and caused the blast. Experts agree.

They point to evidence found after the attack of explosives linked to the cone-shaped devices that can be nailed or magnetically attached to the metal hull of a vessel.

"When we observe the evidence, it is not something caused by an object impacting the vessel," says Jean-Louis Vichot, a former director of the French Naval Academy and retired vice admiral.

"It is in fact a footprint of a limpet mine, one that has not exploded," he said.

Story continues

"The crew spoke of a drone... maybe a device sent on a reconnaissance mission," Vichot added.

- Anti-drone systems -

The possibility that an observation drone was launched to track a future target, formally identify it and monitor its surroundings, is also shared by the former head of a French intelligence service who declined to be named.

"One or several drones could have been used to surveil the most vulnerable vessels," he told AFP.

"It is a classic technique of asymmetrical warfare," the expert added.

Shortly after the drone's initial flight, the assailants must have approached the tanker from behind -- to avoid being spotted by the crew who were on deck -- and discreetly planted the mines before disappearing, experts said.

This would mean that the crew on board the ill-fated tanker could have, in all good faith, thought that the "flying object" they had seen overhead had indeed hit their vessel.

But the former intelligence chief said drones could in the long term be a key device used by assailants to carry out attacks, which will prompt the adoption of a series of security and defensive measures to protect shipping.

He also noted that Iran "considers itself among the world's top five drone powers".

Systems to detect and neutralise drones exist but their performance has yet to be tested efficiently, and their cost for civilian shipping is exorbitant, experts say.