Yemen’s Houthi rebels targeted United States-flagged container ship in the Gulf of Aden on Tuesday, officials said, the latest assault by the group on the crucial maritime trade route.

The captain of the ship reported an explosion in close proximity to the vessel off the coast of Nishtun, Yemen, close to the country’s border with Oman, the British military’s United Kingdom Maritime Trade Operations center said. The Joint Maritime Information Center, which is overseen by the U.S. Navy, identified the ship as the Maersk Sentosa.

The explosion took place in the farthest reaches of the waterway earlier targeted by the rebels, the center said.

It did not elaborate on what caused the explosion, though the Houthis have been known to use drones and missiles as well as bomb-carrying drone boats.

Late Tuesday night, the Houthis issued a broad claim of responsibility for three attacks, which included the Maersk Sentosa. Maersk, a Danish firm which is the world’s biggest shipping company, did not immediately respond for comment. Since the latest assault, shipping or military authorities have not acknowledged any additional attacks in the region.

The last reported Houthi attack in the region took place June 28.

The rebels have targeted more than 70 vessels by firing missiles and drones in their campaign that has killed a total of four sailors. They seized one vessel and sank two since November.

In June, the number of Houthi attacks on merchant vessels increased to levels not seen since December, according to the JMIC. U.S.-led airstrikes have targeted the Houthis since January, with a series of strikes on May 30 killing at least 16 people and wounding 42 others, the rebels say.

The Houthis maintain that their attacks target ships linked to Israel, the United States or Britain as part of rebel support for the militant group Hamas in its war against Israel in the Gaza Strip. However, many of the ships attacked have little or no connection to the Israel-Hamas war — including some bound for Iran.

On June 28, five missiles landed near a Liberian-flagged tanker, Delinox, as it traveled off the coast of the rebel-held port city of Hodeida, according to the JMIC. The following day, Houthi military spokesperson Brig. Gen. Yahya Saree said the group was responsible for two attacks on ships in the Red Sea, but it wasn’t immediately clear which ship was the one reported by the information center.

The Houthis also said they used a drone boat in a June 27 attack on a Malta-flagged bulk carrier, Seajoy.

Meanwhile on Tuesday, the office of the United Nations High Commissioner for Human Rights expressed concern over 13 U.N. staffers and other aid workers who remain detained by the Houthi rebels and called for their immediate release.

“We remain extremely worried about the well-being of 13 U.N. staff and a number of NGO employees who have been detained for over a month now by the ‘Ansar Allah’ de facto authorities in Yemen. We continue to be refused access to them,” the office said in a statement.

Of the 13 employees, the U.N. has said six work for the U.N.’s human rights agency.

Also on Tuesday, the U.S. Central Command issued a statement that its forces destroyed an uncrewed Houthi aerial vehicle in a Houthi-controlled area of Yemen.

(AP)