WASHINGTON (AP) — An armed unmanned surface vessel launched from Houthi-controlled Yemen got within a “couple of miles” of U.S. Navy and commercial vessels in the Red Sea before detonating on Thursday, just hours after the White House and a host of partner nations issued a “final warning” to the Iran-backed militia group to cease the attacks or face potential military action.

Vice Admiral Brad Cooper, the head of U.S. Navy operations in the Middle East, said it was the first time the Houthis had used an unmanned surface vessel, or USV, since their harassment of commercial ships in the Red Sea began after the outbreak of the Israel-Hamas war. They have, however, used them in years past.

Fabian Hinz, a missile expert and research fellow at the International Institute for Strategic Studies, said the USV’s are a key part of the Houthi maritime arsenal and were used during previous battles against the Saudi coalition forces that intervened in Yemen’s war. They have regularly been used as suicide drone boats that explode upon impact.

Most of the Houthis’ USVs are likely assembled in Yemen but often fitted with components made in Iran, such as computerized guidance systems, Hinz said.

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Since late October, the Houthis have launched scores of one-way attack drones and missiles at commercial vessels transiting the Red Sea. U.S. Navy warships have also intercepted ballistic missiles the Pentagon says were headed toward Israel. Cooper said a total of 61 missiles and drones have been shot down by U.S. warships.

In response to the Houthi attacks, Defense Secretary Lloyd Austin in December announced Operation Prosperity Guardian, with the United States and other countries sending additional ships to the southern Red Sea to provide protection for commercial vessels passing through the critical Bab el-Mandeb Strait.

Cooper said 1,500 commercial ships have been able to transit safely since the operation was launched on Dec. 18.

However, the Houthis have continued to launch missiles and attack drones, prompting the White House and 12 allies to issue what amounted to a final warning Wednesday to cease their attacks on vessels in the Red Sea or face potential targeted military action.

Cooper said Operation Prosperity Guardian was solely defensive in nature and separate from any military action the U.S. might take if the Houthi attacks continue.

The U.S., United Kingdom and France are providing most of the warships now, and Greece and Denmark will also be providing vessels, he said.

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Associated Press writer Jack Jeffery in London contributed to this report.