The United States, which has been anxious not to acquire the legal status of "co-belligerent" in the Yemen war, yesterday fired Tomahawk cruise missiles at three radar stations on Houthi-controlled Yemeni territory. The strikes signal a further escalation of tensions in the Red Sea which have been growing over the last fortnight.

Let's recap on how this has developed ...

In the early hours of October 1, Houthi-Saleh fighters attacked a vessel in the Red Sea, apparently firing shoulder-launched rockets from small boats under cover of darkness. The vessel, on lease to the armed forces of the UAE (part of the Saudi-led coalition against the Houthis), was severely damaged but no deaths were reported.

Houthi media said the vessel, HSV-2 Swift, had been doing "logistical work" on behalf of the coalition and was attacked "off the coast of Mokha city" as it "attempted to advance" towards the coast. Mokha is among the Houthi-controlled parts of Yemen.

Exactly what the Swift was doing is still unclear. The UAE says it was on a humanitarian mission to and from Aden, a southern Yemeni port which is outside Houthi control. The Swift was unarmed and its crew was largely – perhaps entirely – civilian. However, it had been built and previously used for military purposes, as a fast logistics vessel. Before arriving in the Emirates in 2013 it had spent 10 years in the service of the US Navy. One of the things it had been designed for was landing troops and heavy equipment from shallow water.

Shipping records show that during the last year or so the Swift had made numerous trips from Assab in Eritrea (where the UAE has a logistics base on the Red Sea facing Yemen) to Aden and another southern Yemeni port, Mukalla.

Assab lies about 60 km south-west of Mokha so a routine voyage between Assab and Aden should not have taken it as far north as the Mokha area. In short, either the Houthis are making false claims about how close the Swift had come at the time of the attack or the Swift was not doing what the Emiratis say it was doing.

Locations of Assab, Mokha/Mocha and Aden. Click here to enlarge map.

The United States responded almost immediately to the attack on the Swift by sending three warships to the area: USS Mason,  USS Nitze and USS Ponce. The Mason and Nitze reportedly carry Tomahawk land attack cruise missiles and Harpoon anti-ship missiles, while the Ponce is described as "a forward staging vessel" and "a mobile base for special operations forces". 

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

The announcement (in Arabic) specifically mentioned the Houthis' attack on the Swift – presumably as an indication of what might happen to others that defied the "warning". 

Militarily, though, the Houthi-Saleh forces don't have the capacity to systematically control what they regard as "unauthorised" shipping, but they are still capable of causing disruption in the Red Sea with sporadic attacks.

The attack on the Swift was a dramatic example of that, but the enormous amount of damage was probably due more to the vessel's construction than the destructive power of the Houthis' weapons. Unlike conventional warships it had a lightweight aluminium hull so delicate that in the words of one of its previous commanders, it would "crinkle like a beer can" if nudged by a tugboat.

By way of background, it's important to note that since the early stages of the conflict the Houthi-controlled areas of Yemen have been subjected to a maritime blockade. Although aimed primarily at depriving the Houthis of weapons it has also prevented food supplies from reaching Yemen and the Houthis have often complained about Yemeni fishing boats being attacked.

On October 8, and initially unrelated to  developments in the Red Sea, the Saudi-led coalition bombed a funeral gathering in Yemen's Houthi-controlled capital, Sanaa, killing at least 140 people and injuring more than 500 – almost certainly the highest civilian casualty toll for any attack during the war so far.

Local indignation over the funeral massacre has been directed at both the Saudis and the Americans – since fragments from the scene indicate that at least one of the bombs used in the attack had been supplied by the United States.

This in turn may help to explain what happened next in the Red Sea.

On the evening of October 9, according to US Navy Captain Jeff Davis, a Pentagon spokesman, a missile was launched from a coastal area of Yemen while the USS Mason was at least 12 miles offshore in international waters. The ship took undisclosed defensive measures and the projectile fell into the sea without causing damage.

A second missile is said to have been fired at the USS Mason about an hour later. According the the US, "it travelled at least 24 miles as the ship was moving away from the shore" and fell into the sea of its own accord without any defensive measures having been taken.

Although this might seem an obvious reprisal for the funeral attack, the Houthis (rather improbably) are denying responsibility. In a statement issued today they suggest the US invented the story of the missiles to create "false justifications to pave the way for Saudi-led coalition to escalate their aggression".

According to the US, a third missile from Houthi-controlled Yemeni territory was launched towards the USS Mason around 6pm on Wednesday. The US then responded overnight with attacks on Yemeni land-based radar stations.

If it's true that the Houthis were taking pot-shots at the US Navy (and there's little reason to doubt it), they could scarcely have chosen a more inopportune moment. The Saudis' funeral bombing had prompted unusually outspoken comments from the Americans and threatened to weaken their support for the kingdom – to the Houthis' advantage. 

Little more than 24 hours later, the targeting of US warships appeared to be driving the Americans and Saudis together again by raising the spectre of Iranian involvement. American news media began hinting – on the basis of very little evidence – that the missiles aimed at the US Navy had been supplied to the Houthis by Iran.

For the Americans, meanwhile, the use of cruise missiles on mainland Yemen – if only for a limited purpose – takes their involvement a notch further. Earlier this week Reuters reported debate within the Obama administration about whether the US can be considered a "co-belligerent" in Yemen under international. The issue had been discussed before the latest developments in the Red Sea and, according to Reuters, lawyers were unable to reach a conclusion. 

As a "co-belligerent" the US would be obliged to investigate allegations of war crimes in Yemen and US military personnel might be exposed to prosecution.

The US will no doubt argue that its action in the Red Sea is not part of the Yemen war and is all about protecting the free movement of shipping in international waters. But in doing so it is treading a very fine line.