The American description of the ship's seizure conflicted in some instances with an earlier account provided by a separate Saudi-led coalition battling Yemen's Shiite rebels, which claimed it had foiled the smuggling attempt. The Saudi coalition alleged that Iran was using the vessel to ship arms to the rebels.
The US Navy's 5th Fleet, based in Bahrain, said a member of the Combined Maritime Forces, a longstanding multinational coalition, intercepted the vessel in international waters last Friday.
A search of the ship determined that it was "stateless," or not formally registered to any country, although it appears to have been coming from Iran, according to the US Navy.
"Based on statements from the dhow's crew, the port of origin of the dhow and its illicit weapons cache is believed to be Iran," the Navy said, adding that the weapons included anti-tank arms thought to be of Iranian and Russian origin.
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Most of the weapons were dumped into the sea, though some were retained for further analysis by sailors aboard the American warship.
Earlier in the day, the Saudi-led coalition claimed it had foiled the smuggling effort, which it said happened a day later than in the American account, some 240 kilometers (150 miles) southeast of the Omani port of Salalah.
Saudi Arabia and its coalition allies fear that Iran is actively providing aid to the Yemeni rebels, known as Houthis, as a way to gain a foothold on the Arabian Peninsula. Iran acknowledges providing political support to the rebels but denies arming them directly.
Cmdr Kevin Stephens, a spokesman for the US 5th Fleet, declined to identify the country that made the initial interception but confirmed it was not one of the Gulf nations in the Saudi-led coalition.