Despite several rounds of US-led air strikes in Yemen, experts say there is no certainty these are deterring the Houthis or giving nervous mariners the reassurance they need.
As the Houthis use drones to attack vessels, Iran is cashing in on a boom in unmanned aerial vehicles as recent conflicts show their wide range of uses, according to an annual survey of global military might.
The Military Balance report by the International Institute of Strategic Studies in Britain warns of a “more dangerous decade” ahead, with global military spending rising to $2.2 trillion in the face of an unstable world order.
The fighting between Israel and Hamas and the Houthi missile threat are two contributing factors to a “highly volatile security environment”, said the report launched in London on Tuesday.
Anti-Houthi air strikes carried out by the US and Britain are intended to limit the threat from the Yemeni militants by destroying their military capability and deterring further attacks on shipping.
A third round of joint US-UK strikes on February 4 attacked buried weapons stores, missile launchers, radars and air defence systems used by the Houthis, according to Pentagon officials.
While the number of attacks on commercial shipping has dropped in February, it is difficult to know whether this means the Houthi arms stockpile has taken a hit in the US-UK campaign, analysts said.
“These are very difficult targets to get at,” said IISS expert Nick Childs. "They are very mobile, easily hidden. There need to be very granular degrees of intelligence, surveillance and reconnaissance. Clearly, it hasn’t fully delivered.
“There are question marks over whether there’s a more sporadic threat at the moment and whether that is, as has been claimed, because there has been a degrading of capability. That’s, I think, unclear.”
A new Houthi attack was reported overnight on a Greek-owned cargo ship carrying corn from Brazil to Iran, with two missiles fired from a rebel-held part of Yemen, according to the US military.
The Houthis “have gained quite a lot of experience in hiding their missile capabilities during their years of war against the Saudi and Emirati-led coalition”, said IISS researcher Fabian Hinz.
The militants may have smuggled in Iranian weapons over land, in small ships, or built their own components, Mr Hinz said – with some Houthi designs looking “a lot like they could be local production” with advice from Iran.
Iranian “guidance kits” are believed to have been used to convert old Soviet missiles, according to the institute’s research. While the Houthis may not make cruise missile engines, these are regarded as relatively easy to smuggle.
On drones, the Military Balance study says recent wars “have demonstrated the utility of a far greater range of such systems, such as direct-attack munitions, quadcopters, and more traditional medium and high-altitude platforms”.
“Demand has spurred a wave of export deals, with Turkey and Iran providing UAVs to various actors,” says the report, which claims Tehran has the biggest military manpower in the Middle East.
The Houthis have said they “will not hesitate to carry out more operations” in solidarity with Hamas’s war against Israel, as well as in retaliation to the US-UK strikes.
Moreover, Mr Hinz said, even a reduced Houthi threat level that is acceptable to the US navy may still be too high for commercial ships, which have taken costly diversions around the southern tip of Africa.
“If you manage to degrade the Houthi arsenal substantially, and then you still have defences that have proven to work very well on US Navy warships, that is a quite favourable outcome from a purely military point of view,” he said.
But “if you look at the risk tolerance of these shipping companies, basically you would have to degrade the arsenal entirely, and that is something that’s just not possible”.
Ben Barry, a former British Army brigadier, meanwhile expressed concern that a misfiring western missile or an “over-enthusiastic” Iran-backed group could escalate the situation in the Middle East.
Retaliatory US strikes against Iranian allies in Iraq and Syria have added to the complex picture in the region, as the war between Israel and Hamas rages on into its fifth month.
“All these organisations have quite a degree of autonomy and may not necessarily be doing what Iran wants them to do all of the time,” Brig Barry said.
“Despite the very high degree of precision of many western weapons, there’s still the possibility of one going wrong and there’s still the possibility of an intelligence error, a wedding party being hit or more collateral damage than anticipated, which could then create an action-reaction cycle.
“So the longer the main war, the Israel-Gaza war goes on, the more chance there is that one of the secondary conflicts could accidentally spin out of control.”