More than 70 tonnes of ammonium perchlorate, a powerful oxidiser commonly used to make rocket and missile fuel, were found, as well as explosives, the Navy said.
The US 5th Fleet stopped the vessel and its four Yemeni crew in international waters on November 8. It was the fleet’s first ever ammonium perchlorate discovery.
“This was a massive amount of explosive material, enough to fuel more than a dozen medium-range ballistic missiles depending on the size,” said Vice Adm Brad Cooper, commander of US Naval Forces Central Command, US 5th Fleet and Combined Maritime Forces.
“The unlawful transfer of lethal aid from Iran does not go unnoticed. It is irresponsible, dangerous and leads to violence and instability across the Middle East.”
The forces also found more than 100 tonnes of urea fertiliser, a chemical compound with agricultural applications that is also uses as an explosive precursor.
“The direct or indirect supply, sale or transfer of weapons to the Houthis violates UN Security Council Resolution 2216 and international law,” Vice Adm Cooper said.
US 5th Fleet previously seized 40 tonnes of urea fertiliser on January 18 on another fishing vessel in the Gulf of Oman that had attempted to smuggle illicit weapons off the coast of Somalia months earlier.
The Houthis seized Yemen’s capital, Sanaa, in September 2014 and forced the internationally recognised government into exile. A Saudi-led coalition entered the war on the side of Yemen’s exiled government in March 2015.
Years of inconclusive fighting has pushed the Arab world’s poorest nation to the brink of famine.
A UN arms embargo has prohibited weapons transfers to the Houthis since 2014. Despite that, Iran has transferred rifles, rocket-propelled grenades, missiles and other weaponry to the Houthis via dhow shipments.
Iran denies arming the Houthis. But independent experts, western nations and UN experts have traced components seized from detained vessels back to Iran.
A six-month ceasefire in Yemen’s war, the longest of the conflict, expired in October despite diplomatic efforts to renew it. That has led to fears fighting could again escalate. More than 150,000 people have been killed in the war, including more than 14,500 civilians.