Limpet mines, which appear to be the cause of damage to at least one tanker attacked in Gulf of Oman last week, are classic maritime weapons dating back to World War II.
The cone-shaped devices can be nailed or magnetically attached to the metal hull of vessels.
Like sea limpets -- the creatures that latch on to rocks -- these mines can cling onto metal.
"The first use of these mines was by Italian combat divers who attacked a British fleet in the Mediterranean" at the start of World War I, said Jean-Louis Vichot, a former director of the French Naval Academy.
"They are usually attached by combat divers, and this is apparently what happened last month to the four ships" targeted in a sabotage attack off the UAE port of Fujairah on the Gulf of Oman, he told AFP.
"The explosions that rocked the ships happened under water."
In 1942, 10 British commandos famously used limpet mines to sabotage merchant ships in Nazi-occupied Bordeaux, in southwestern France. Only two of the commandos survived.
Frogmen working for French intelligence also used limpet mines to sink the Greenpeace flagship Rainbow Warrior when it was docked in Auckland, New Zealand, in 1985.
Last week, limpet mines were placed above the water level on the hull of the Japanese-owned Kokuka Courageous as it was passing through the Gulf of Oman along with the Norwegian-operated Front Altair, according the US Navy.
Their placement indicated the job was not done by divers, said Vichot.
Ludovic Guerineau, a former member of the French external security service, agreed.
"In the case of the Japanese tanker, the (limpet mines) were placed by individuals on a boat," he told AFP.
"It was most probably a speedboat that discreetly approached (the ship) from behind, undetected by radar, and planted them before disappearing."
Vichot said it was not surprising the Kokuka Courageous's crew did not see the culprits plant the mines, instead reporting that they had seen a "flying object".
"The concern of crew members of a tanker this size is a collision with other boats and getting to shore," he said, adding there is usually one or two watchmen only, sometimes none.
"Also, when they pass through the Strait of Hormuz, they are accustomed to seeing speedboats passing by and sometimes even doing strange manoeuvres.
"That said, however, ship owners now must have given them different instructions."
- 'Scare people' -
The two experts said the limpet mines were used to merely cause damage.
"It was a message during a time of high regional tensions," said Guerineau, in reference to the strains between regional arch-rivals Saudi Arabia and Iran.
On May 12, four tankers, two of them Saudi, were damaged in a "sabotage attack" off the port of Fujairah.
The United States and Saudi Arabia have accused Iran of being behind the attacks.
"What they are trying to is scare people and increase insurance premiums, and they succeeded," said Vichot.
The US Navy said Wednesday the limpet mine used in the attack on the Kokuka Courageous was "distinguishable and it is also strikingly bearing a resemblance to Iranian mines that have already been publicly displayed in Iranian military parades".
Tensions between the United States and Iran have grown since the US last year unilaterally quit the multilateral 2015 nuclear deal and reimposed sweeping sanctions on the Islamic republic.
The United States has since bolstered its military presence in the Middle East and blacklisted Iran's Revolutionary Guard as a "terrorist organisation".