An armed unmanned surface vessel launched from Houthi-controlled Yemen got within a “couple of miles” of U.S. Navy and commercial vessels 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 conflict. 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.