Houthi-Saleh forces – the de facto government in the north of Yemen – have "warned" foreign ships not to enter the country's territorial waters without "prior authorisation from the competent Yemeni authorities". 

The announcement, circulated by the Houthi-controlled Saba news agency on Monday, sounds like a threat to cut off seaborne supplies to the southern-based rival government led by Abd-Rabbu Mansour Hadi and its military backers. The "warning" may be largely bluster but the US has responded by dispatching three warships to the area.

In the early stages of the conflict the Saudi-led coalition, which regards Hadi as Yemen's legitimate president, imposed a naval blockade in the Red Sea aimed at depriving Houthi-controlled areas of weapons. This has also prevented food supplies from reaching Yemen and the Houthis have often complained about Yemeni fishing boats being attacked.

Although the Houthi-Saleh forces have little prospect of imposing a similar blockade on Hadi and his supporters they have already shown themselves capable of causing some disruption.

On Friday/Saturday night near the Bab-al-Mandab strait, Houthi-Saleh fighters hit a vessel controlled by the Emirati military who are part of the Saudi-led coalition. The attackers are thought to have fired shoulder-launched rockets from small boats under cover of darkness.

The UAE's government news agency initially reported that the vessel had been involved in "an accident" which caused no injuries. Houthi sources, quoted by Hizbullah's al-Manar TV, claimed  the vessel was an "Emirati warship" which had been "completely destroyed".

A video circulated on the internet shows the vessel at sea during daylight followed by a series of explosions at night, but it is impossible to know whether this is a film of the actual attack.

The vessel concerned, HSV-2 Swift, was built in Australia in 2003 with a very unusual design. It is described in Wikipedia as "a wave-piercing, aluminium-hulled, commercial catamaran with military enhancements, such as a helicopter flight deck, vehicle deck, small boat and unmanned vehicle launch and recovery capability, and a communications suite". 

This construction, together with its lack of water-tight compartments, probably meant the Houthis could inflict more serious damage on the Swift using basic weapons than they could on regular warships. One of its previous commanders was quoted as saying: "We're aluminum, so we crinkle like a beer can if a tug pushes up against us." 

Until 2013 it was leased to the US Navy for evaluation purposes and used mainly as a support vessel and for humanitarian missions. More recently it has been leased to the Emirati military, plying frequently between Eritrea and the Yemen ports of Aden and Mukalla.

The Saudi-led coalition has now denounced the attack on the Swift as an act of terrorism against a "civilian" ship in international waters. 

According to the coalition, the Swift was was being used to "transfer relief and medical aid [to Yemen] and evacuate wounded civilians". The coalition also says it was carrying civilian passengers at the time of the attack. However, there is no independent confirmation of these claims and it's unclear why, if the vessel was used purely for civilian purposes, it was in the hands of the military.

Video showing the Swift in operation when it was leased to the US Navy