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Yemeni security forces halted and searched the boat, a 130-foot dhow, last Tuesday and found the weapons in three large cargo rooms in the hold, according to reports on the mission reaching Washington. There was American support for the interdiction, officials said.

The government of Yemen confirmed the seizure Monday in an official statement. The captured weapons included surface-to-air missiles used to shoot down civilian and military aircraft, C4 military-grade explosives, 122-millimeter shells, rocket-propelled grenades and bomb-making equipment, including electronic circuits, remote triggers and other hand-held explosives, the statement said. If the weapons turn out to be the Iranian-made Misagh-2, as cited in the reports from Yemen, it would reflect a significant increase in lethality for the insurgents.

Yemen is already awash with small arms and explosives acquired over years of war and insurgency, much of it brought in from a number of foreign sources through its poorly controlled ports. There has been little effort to regulate the supply – one governor of a northern province is also a major arms dealer – and insurgents have often raided the stores of Yemen's corrupt and divided military. Many of Yemen's unruly tribes command powerful arsenals.

The United States has a publicly acknowledged security assistance effort under way with Yemen. At the same time, the American military and the C.I.A. are engaged in a clandestine program of using drones to strike militants associated with a terrorist organization, Al Qaeda in the Arabian Peninsula, based in Yemen. With the United States and Saudi Arabia providing both public and secret security assistance there, and with Iran also said to be arming militant forces, Yemen has become the battlefield for a major proxy war by outside powers.

American officials said the weapons on board were made in Iran, and that the pattern of shipment aboard the boat matched past instances of suspected Iranian smuggling into Yemen. Officials described the smuggling as part of a plan by Iran to increase its political outreach to rebels and other political figures in Yemen. To identify with greater certainty the source of the seized weapons, the boat's navigation instruments will most likely be examined to determine its origin and route, and the crew will be questioned.

For years, Yemen has accused Iran of supporting the Houthi rebels, who fought an intermittent guerrilla war against the Yemeni government from 2004 to 2010. Those accusations – including claims of intercepted weapons shipments – often lacked evidence and, up until about a year ago, routinely were dismissed as propaganda.

But after the uprising in Yemen in 2011, the Houthi movement expanded from its base in the northwest — now a de facto Houthi statelet — across the country. It has benefited from widespread dissatisfaction with both Yemen's government and the local equivalent of the Muslim Brotherhood, known as Islah.

By last spring, American military and intelligence officials described what they viewed as a widening effort to extend Iranian influence across the greater Middle East.

Iranian smugglers backed by the Quds Force, an elite international operations unit within Iran's Islamic Revolutionary Guards Corps, had begun shipping AK-47s, rocket-propelled grenades and other arms to replace older weapons used by the rebels, American officials said early last year.

Using intercepted cellphone conversations between the smugglers and Quds Force operatives provided by the Americans, Yemeni and Indian coastal authorities seized some smaller shipments, according to American and Indian officials.