Credible evidence for Iran arming or funding the Houthi movement in Yemen?

Question

Reuters carries this news item, beginning with the following paragraph:

Iran is sending advanced weapons and military advisers to Yemen’s rebel Houthi movement, stepping up support for its Shi’ite ally in a civil war whose outcome could sway the balance of power in the Middle East, regional and Western sources say.

(emphasis mine.)

I'm very jaded and skeptical about claims by the US and its allied regarding use of arms by middle-east regimes they don't like (e.g. Iraq WMDs). The US has been quite hostile to Iran, especially with the Trump administration reneging on the JCPOA agreement and their amped-up rhetoric against the Iranian regime. The US is also allied to Saudi Arabia, which it is arming and otherwise supporting in its military campaign in/against Yemen; and Iran is kind of a nemesis for SA (perhaps also in the Sunni-vs-Shia aspect).

So, you can understand my skepticism. But just because some parties are biased does not mean their claims are invalid. What credible evidence is there, if any, for the claims of funding and arming by Iran of the Yemeni Houthi movement?

Notes:

• I'm differentiating arms or funding from diplomatic support, rhetorical encouragement, favorable trading conditions, freedom to conduct foreign affairs etc - which are also a kind of support, but of different significance/intensity.
• See also the relevant section of the Wikipedia article on the Houthis.

Answer 1

Most western governments claim to have evidence of Iran military involvement in Yemen, including weapons shipments, that they keep classified.

However, here are some concrete elements of proof that have reached the world's medias. How decisive they are is up to you; Western governements', Saudi's and Iran's reactions about them have widely differed.

2013 : Ship Jihan I

Yemeni authorities have seized an Iranian boat full of weapons:

Yemeni authorities point to the “Jihan 1” as evidence of Iran’s support. The ship was seized by Yemen in 2013, smuggling weapons from Iran to local insurgents. The Yemeni official showed Reuters a breakdown of the cargo, which included Katyusha rockets M-122, heat-seeking surface-to-air missiles, RPG-7s, Iranian-made night vision goggles and “artillery systems that track land and navy targets 40km away”.

But of course:

Iran denied any connection with the arms found on Jihan 1.

2015 : UN 'secret' report

In April 2015, a report by UN expert was transfered to the Iran Sanction Committee but has leaked to the medias (notably French AFP) and been widely reported.

Studying evidence that includes afore-mentioned Jihan I, it suggests that weapons were delivered by sea from Iran to Yemen "at least since 2009":

Les informations recueillies "suggèrent que le cas du Jihan suit d'autres livraisons par mer au Yémen que l'on peut faire remonter au moins à 2009", indique le rapport que l'AFP a pu consulter.

In English:

The collected informations "suggest that the Jihan case follows other shipments by sea to Yemen that can be dated back at least to 2009", says the rapport AFP could consult. - my translation.

2017: new UN report

On dec 1, 2017, Reuters says it has had access to another report, dated nov 24, 2017:

Remnants of four ballistic missiles fired into Saudi Arabia by Yemen’s Houthi rebels this year appear to have been designed and manufactured by Riyadh’s regional rival Iran, a confidential report by United Nations sanctions monitors said.

The existence of the report was confirmed on dec 13, 2017, when the UN Secretary General Antonio

Guterres said in a report to the security council that the United Nations was investigating Iran’s possible transfer of ballistic missiles to Houthi Shia rebels in Yemen that may have been used in launches aimed at Saudi Arabia in July and November.

2017 : Remnants of ballistic missiles shown in Bolling Air Force base in Washington.

On Nov,14,2017, Nicky Haley presented short-range ballistic missiles that are said to have been built in Iran, delivered to Houthis and fired in Yemen.

Standing in front of segments of two missiles, which US officials say were fired recently by Houthi forces at Saudi Arabia, Haley said: “As you know we do not often declassify this time of military equipment recovered from these attacks but today we are taking an extraordinary step of presenting it here in an opening setting.”

“In this warehouse is concrete evidence of illegal Iranian weapons proliferation gathered by direct military attacks on our partners in the regime,” she added, saying that representatives from other countries had been invited to inspect the evidence at Bolling.

Sure, US officials displaying proofs of enemy weapons is taken with a bit of salt at least since Colin Powell... Guess what Iran's comment has been ?

The Iranian spokesman at the UN, Alireza Miryousefi, said the evidence was fake.

He said: “We categorically reject it as unfounded and, at the same time, irresponsible, provocative and destructive. This purported evidence, put on public display today, is as much fabricated as the one presented on some other occasions earlier.”

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Edit Feb 2023: A large shipment of Iranian weapons has been captured by a French effort supported by the US on Jan 15, 2023.

The French operation is the latest in a series of weapons seizures that suggest that Iran continues to supply its Houthi allies in Yemen with firepower

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Answer 2

A recent study by the Global Initiative Against Transnational Organized Crime claims that Iranian weapons made it to Somalia via the Iran-Yemen route. According to its report entitled AN IRANIAN FINGERPRINT? Tracing Type 56-1 assault rifles in Somalia:

As the GI-TOC has previously reported, at least one Somalia-based trafficking network is intricately involved in the maritime transfer of SALW [Small Arms Light Weapons] from Iran to the Houthis. However, over the course of the current study the GI-TOC has established for the first time that weapons originating in the Iran–Yemen arms trade are being trafficked onward into Somalia itself. Over the course of eight months, GI-TOC researchers documented over 400 illicit weapons in 13 locations across Somalia. The documented materiel included 38 Type 56-1 assault rifles – Chinese-manufactured AK-pattern rifles – that had likely originated in Iranian arms shipments to the Houthis. The majority of Type 56-1 rifles documented in this study were found in Puntland, a semi-autonomous region in northern Somalia which has historically functioned as the gateway for illicit weapons into the country. However, the GI-TOC documented Type 56-1 rifles as far south in Somalia as Dolow, a town bordering Ethiopia.

The above quote lays out the accusation in the study, namely that Chinese-made weapons were trafficked from Iran to Somalia via the Iran-Yemen route. The study is based around an analysis of the serial numbers, but it provides more information that points to Iranian involvement (quotes may be somewhat out of context, but I'm trying to provide the relevant excerpts from the report):

In a January 2021 report, UN sanctions monitors cited testimony from an arrested Yemeni arms trafficker who claimed to have received maritime training in Iran and to have subsequently participated in several trafficking operations, during which weapons originating in Iran were trans-shipped off the coasts of Oman, Djibouti and Somalia. The alleged trafficker further stated that the cargo would then be transported either to ports in Al Mahrah Governorate, where another network would smuggle them overland to Houthi-controlled areas, or directly through the Bab-el-Mandeb strait to ports on the Red Sea.

Ascertaining Iranian involvement in these trafficking operations is difficult, but three seizures carried out by the USS Monterey (2021), the USS Jason Dunham (2018) and the USS Winston Churchill (2021) all provide strong circumstantial evidence.

Each of these seizures is then discussed in the report, including inventories and pictures of the seizures. I will not repeat those here.

Based on comparing serial numbers of the weapons found in Somalia and those in one of the seizures, the report determines that they likely come from the same batch:

The serial number sequence strongly suggests that the rifles were issued from a common source, in this case Iranian government stocks. However, without more information on the distribution pattern of these rifles, it is difficult to determine when and by what means the rifles reached Somalia. In most cases, it is open to speculation whether they were transshipped from Iranian consignments en route to Yemen, or diverted into the illicit arms market only after reaching Yemen.

Then the report discusses a case of two weapons with a certain stamp (the first in one of the US seizures, the other was found by the researchers in Somalia):

Notably, the rifle bore a ‘21 SEP’ post-manufacturing marking, making it highly likely that it had once formed part of a Houthi arsenal. (The date 21 September has special significance to the Houthi movement, marking the date in 2014 when Houthi militants stormed and took control of the Yemeni capital of Sana’a.) The above rifle therefore probably reached its intended end users, namely the Houthis, before being trafficked onward to Somalia.

One other rifle bearing the ‘21 SEP’ stamp was observed by GI-TOC field researchers. The weapon, a Type 56-1 rifle documented in the possession of a civilian resident of Galkayo in January 2021, does not appear to have originated from the same Iran–Yemen supply chain as other Type 56-1s referenced in this study. However, the serial number and factory marking on the weapon bore some similarities with rifles captured during the 2015 interdiction of a skiff within Iran’s EEZ, operated by individuals who later claimed to have received training at an slamic Revolutionary Guards Corps base in Iran.29 The materiel captured as a result of this interdiction was reportedly intended for anti-government forces in Bahrain.

There may be some more relevant evidence in the report, but I think this already gives a sufficient indication of their approach and their reasoning. I'll end with relevant excerpts from Reuters' reporting on the study:

The Global Initiative against Transnational Organized Crime said its study drew on data from more than 400 weapons documented in 13 locations across Somalia over eight months and inventories from 13 dhows intercepted by naval vessels.

It is the first publicly available research into the scale of illicit arms smuggling from Yemen into the Horn of Africa country.

"Weapons originating in the Iran–Yemen arms trade are being trafficked onward into Somalia itself," said the study, which is due to be published on Wednesday.

And Reuters' summary of the reasoning in the report:

The study said the investigators were not able to fully document the buyers and sellers of the weapons.

But it said signs the weapons were originally supplied by the Iranian state included serial numbers that were very close together, indicating they were part of the same shipment, information from satellite navigation systems on seized dhows and human intelligence from trafficking gangs.

One dhow carrying weapons which was seized by a U.S. navy vessel had a GPS with stored points in Iran, southern Yemen and Somalia, the report said, including a small anchorage near Jask port, which hosts an Iranian naval base, and "home" as the Yemeni port of Mukalla, a well-known arms smuggling hub.