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 last year.

Senior officials briefed on the mission said that the Yemeni Coast Guard had conducted the operation jointly with American military 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.

American intelligence played a role in the seizure, most importantly in pinpointing the vessel from among the large numbers of traditional fishing and cargo boats sailing in and out of Yemeni waters. Officials declined to describe the intelligence that identified the vessel, except to say that it was moving erratically and sitting low in the water, and that various standard techniques, like human intelligence, overhead surveillance and communications analysis, went into the effort.

This interdiction comes at an extremely delicate time in Yemen, with the government largely paralyzed, sectarian tensions rising and accusations of Iranian interference — which have long been used as a propaganda tool here — on the increase.

Iran is also accused of supporting the secessionist movement in southern Yemen, which has also held vast public rallies in recent days and now poses a serious threat to the Yemeni government. The most prominent television station in southern Yemen, Aden Live, is run by a Yemeni political figure who has acknowledged receiving funding from Iran.

Iran’s goal in supporting these rebellious political currents, critics say, is to foster a chaotic environment and weak state where it can maintain influence through its allies, much as it does in Lebanon. But analysts caution that such accusations have long been a staple of political discourse in the Gulf, where Sunni governments use them to marshal sectarian animosities against Shiites in Iran and elsewhere.

Bernard Haykel, a professor at Princeton University and an expert on Yemen, said 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.