Guns supplied by Iran to its allied Houthi militias in Yemen are being smuggled across the Gulf of Aden to Somalia, according to a Geneva-based think tank, where al-Qaeda-linked al Shabab insurgents are battling a weak and divided government.
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.
"Iran has repeatedly denied any involvement in the trafficking of arms to the Houthis. However, a preponderance of evidence points to Iranian state supply."
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 US 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.
The study said the guns end up with commercial smuggling networks whose customers can include armed factions seeking advantage ahead of Somalia's repeatedly delayed presidential elections, as well as clan militias and rival extremist insurgent groups linked to al-Qaeda and ISIS.