LONDON: Yemen's Houthis are sending drone boats packed with explosives into the Red Sea as they intensify their attacks on merchant ships that have little defence against the "sophisticated shift" in tactic, maritime security sources say.

Iran-aligned Houthi militants first launched aerial drone and missile strikes on the trade route in November in what they say is solidarity with Palestinians in Gaza.

In over 70 attacks, they have sunk two vessels, seized another and killed at least three seafarers.

In recent weeks, at least three ships have been attacked by unmanned surface vehicles (USVs) and in one incident it contributed to the sinking of the Tutor cargo ship. There was no such activity in November.

"These USVs, loaded with explosives, represent a sophisticated shift in asymmetric warfare tactics, enabling the Houthis to strike with precision and at a distance, thus minimising their exposure to counter-attacks," Dimitris Maniatis, CEO of Maritime Risk Managers MARISKS, said.

According to maritime security sources and Reuters analysis, there have been at least six defensive strikes on USVs by US-led coalition warships since February.

On June 27 and June 30, two separate vessels were targeted by Houthi swarming tactics, including multiple seaborne attack drones, maritime agency, the United Kingdom Maritime Trade Operations (UKMTO), said.

"USVs strike vessels at the waterline, and this, combined with the substantial warhead size, has the potential for significant water ingress and damage control issues," said Munro Anderson, head of operations at marine war risk and insurance specialist Vessel Protect, part of Pen Underwriting.

"It is highly likely that in light of the very public success such devices have had, when used by Ukrainian forces within the Black Sea, that Houthis have sought to deploy such tactics to their own ends."

A Houthi spokesperson did not respond to a request for comment.

In another emerging tactic, some of the USVs have been logged as potentially having dummies to resemble pirates in another psychological tactic aimed at confusing seafarers, an official with Greek maritime security company Diaplous said.

"In most cases we understand that the Houthis are using 'spotters' at sea, who often record the attack from a small distance, and on most (if not all) operations remotely steer the USV to the target," MARISKS Maniatis added.

Insurance industry sources said that additional war risk premiums, paid when vessels sail through the Red Sea, were quoted up to 0.7 per cent of the value of a ship in recent days from around one per cent earlier this year, adding hundreds of thousands of dollars of extra costs and the latest threats could push rates higher in coming weeks.

Rates for Chinese vessels - seen as not having any connection with Israel or the US which are targeted - have remained around 0.2 to 0.3 per cent in contrast, sources added.

 - Reuters