Yemen's Houthi militia launched another attack on shipping lanes in the Red Sea on Thursday morning, the commander of 5th Fleet confirmed to reporters, bringing the total number of attacks since Nov. 18 to 25.

The attack was conducted using a one-way unmanned surface vehicle, Vice Adm. Brad Cooper said, the first attack of this kind since the Houthis began targeting the Red Sea in the fall.

“Fortunately, there were no casualties and no ships were hit, but the introduction of a one-way attack USV is a concern,” he said, adding that it was unclear what the drone was targeting.

A one-way USV can be launched and then loiter in place for hours while operators search for a target.

Around 55 countries have been affected by the attacks so far, Cooper said, either because of the ships’ flags, the origin or destination of the goods on board, or the nationalities of the crew.

And because the attacks are not precise and ships are traveling in such close proximity, “any ship really at any time is at risk of collateral damage when passing through the Houthi controlled territory in the vicinity of the southern Red Sea,” Cooper added.

The multinational response, known as Operation Prosperity Guardian, includes commitments by several countries’ navies to patrolling that part of the Red Sea consistently, Cooper said.

“And together we now have the largest surface and air presence in the southern Red Sea in years,” he said, noting a force that includes five international warships, manned and unmanned reconnaissance aircraft and tactical aircraft from the deployed aircraft carrier Eisenhower.

“Since the start of the operation, there’s been no Houthi UAV or missiles actually fired into international shipping lanes that have actually hit any merchant vessels,” Cooper said.

Some munitions have come close, he added, but 1,500 vessels have been able to safely transit the Bab el-Mandeb strait since the beginning of the operation.

Operation Prosperity Guardian is a purely defense effort, but international leaders have promised retaliation if the attacks do not cease.

“The Houthis’ reckless attacks continue, as you know, and there are no signs that their irresponsible behavior is abating,” Cooper said.