Houthi militants appear to be ignoring U.S. warnings about their repeated attacks on ships in the Red Sea, launching a naval drone Thursday that came within “a couple of miles” of merchant ships and American combat vessels before detonating.

U.S. naval officials said the so-called unmanned surface vessel was launched from Houthi territory in Yemen and traveled about 50 miles into busy shipping lanes before exploding. They said it is unclear who or what the Houthis were trying to target, adding the explosion did not cause any damage or injuries.

“There are no signs that their irresponsible behavior is abating,” Vice Admiral Brad Cooper, head of U.S. Navy operations in the Middle East, told reporters.

“Shipping lanes in this region are dense,” Cooper said. “These Houthi attacks are, for sure, destabilizing and contrary to international law and clearly, as have as has been articulated by many, must stop immediately.”

Twenty-five attacks since November

Since November, the Iranian-backed Houthis have launched 25 attacks on vessels sailing through the Red Sea, claiming the ships are linked to Israel and that the attacks are in support of Palestinians in Gaza.

In response, the United States, along with France, Britain and nearly 20 other countries launched Operation Prosperity Guardian in mid-December to protect ships from Houthi attacks.

So far, about 1,500 vessels have passed safely through the Bab el-Mandeb Strait, with the U.S. and its partners shooting down 11 Houthi drones, six anti-ship ballistic missiles and two cruise missiles. U.S. forces also sunk three Houthi boats Sunday after they attacked a container ship.

FILE - A 20mm Phalanx CIWS weapons defense cannon is mounted on the U.S. Navy destroyer USS Gravely, March 14, 2023. The U.S. military said Sunday it shot down two anti-ship ballistic missiles fired toward a container ship by Yemen’s Houthi rebels in the Red Sea.

FILE – A 20mm Phalanx CIWS weapons defense cannon is mounted on the U.S. Navy destroyer USS Gravely, March 14, 2023. The U.S. military said Sunday it shot down two anti-ship ballistic missiles fired toward a container ship by Yemen’s Houthi rebels in the Red Sea.

“We now have the largest surface and air presence in the southern Red Sea in years,” Cooper said. “And in the coming weeks, we expect additional countries to contribute, which will only strengthen our ability to deter.”

But Cooper also said the five warships and other assets taking part in Operation Prosperity Guardian are “entirely defensive in nature” and are separate from any capabilities that might be used to strike at the Houthis.

US, allies warn Houthis

On Wednesday, the U.S. and 12 allies issued a statement warning the Houthis of unspecified consequences if their attacks on shipping in the Red Sea continue.

“Let our message now be clear: We call for the immediate end of these illegal attacks and release of unlawfully detained vessels and crews,” the statement said.

Signatories on the statement include Britain, which on Monday issued its own warning to the Houthis of “direct action,” as well as Australia, Canada, Germany and Japan.

Late Wednesday, a senior U.S. administration official —briefing reporters on the condition of anonymity — said the Houthis should “not anticipate another warning” from the U.S. or its partners.