The arms intercepted Wednesday aboard the ship off the coast also included military-grade explosives, rocket-propelled grenades and bomb-making equipment, according to a statement from the Yemeni Embassy in Washington.
A U.S. official, speaking on condition of anonymity, confirmed to Reuters that the operation was coordinated with the U.S. Navy and that a Navy destroyer was nearby.
A second official told Reuters the intercepted shipment was believed to have been from Iran and destined for insurgents, likely the Houthis, a Shi’ite movement in the north of the country.
"This demonstrates the ever-pernicious Iranian meddling in other countries in the region," said the second U.S. official, who also spoke on condition of anonymity.
Iran denies any interference in Yemen's affairs.
Analysts and diplomats believe that the ascent of the Houthis, named after their leaders' family, has turned Yemen into a new front in a long struggle between Iran and Western powers and the Arab regimes they support.
Sunni-dominated Arab governments and Sunni clerical allies accuse Iran of backing Shi'ite communities around the region, and Sanaa has also accused Iran of trying to meddle in Yemeni affairs.
Yemen's President Abd-Rabbu Mansour Hadi snubbed a visiting Iranian envoy last year to signal "displeasure" with Tehran after Sanaa said it had uncovered an Iranian-led spy ring in the capital, Reuters reported.
Earlier this month, the U.S. envoy to Yemen, Gerald Feierstein, was quoted accusing Iran of working with southern secessionists as well. Yemen is also grappling with al Qaeda-linked Sunni militants in the north.
Yemen's government said in a statement the shipment was intercepted in Yemeni waters, close to the Arabian Sea. It said Yemeni Coast Guard officials boarded the vessel, which flew multiple flags and had eight Yemeni crew members on board.
Senior officials briefed on the mission told The New York Times that the Yemeni Coast Guard had conducted the operation jointly with American forces. An American boarding party from the Navy destroyer Farragut accompanied the Yemeni crew as it interdicted, boarded, inspected and seized the vessel, officials said.
"Authorities are continuing to investigate the vessel's shipping route by analyzing navigation records found on board the ship," the Yemeni statement said.
If the weapons turn out to include the Iranian-made Misagh-2 surface-to-air missile, as cited in the reports from Yemen, it would reflect a significant increase in lethality for the insurgents, the Times notes.
Bernard Haykel, a professor at Princeton University and an expert on Yemen, told the Times Iran was being “opportunistic” in its support for the Houthis and was trying to counteract the American and Saudi support for Yemen’s government. But Tehran is hardly controlling the group in Yemen, he said.
“Iranians want to needle the Saudis in every possible way,” Haykel said. “But to say that the Houthi are proxies of Iran is stretching the boundaries of credibility.”