For years, Iran has been under a series of international sanctions prohibiting it from exporting arms. The United States has frequently claimed that Tehran has violated the sanctions in support of proxy forces in many conflicts, including in Iraq, Syria, Yemen and the Palestinian territories.
The grenade launchers that were the subject of Mr. Schroeder’s analysis are the central component of a reusable weapon system commonly called RPG-7s.
They were among 81 launchers seized on the Samer by Australian sailors, part of a hidden cargo that included 1,968 Kalashnikov assault rifles, 49 PK machine guns, 41 spare machine-gun barrels and 20 60-millimeter mortar tubes — enough weapons to arm a potent ground force.
Although the evidence was not conclusive, Mr. Schroeder said, “the seizure appears to be yet another example of Iranian weapons being shipped abroad despite longstanding U.N. restrictions on arms transfers from Iran.”
With Iran observing three days of mourning following the death of Ayatollah Ali Akbar Hashemi Rafsanjani, it was not possible to contact the government for comment. But on previous occasions, Iran has refused to respond to inquiries about the smuggling.
The Samer episode was one of four interdictions of Iranian dhows from September 2015 through March 2016 that yielded, in total, more than 80 antitank guided missiles and 5,000 Kalashnikov rifles as well as sniper rifles, machine guns and almost 300 RPG launchers, according to data provided by the United States Navy.
In 2013, the Navy stopped another dhow off the Yemeni coast and found it to be carrying shoulder-fired antiaircraft missiles and launchers, rifle and machine-gun cartridges, C4 plastic explosives, night-vision equipment and other military items.
In an interview in Bahrain, Vice Adm. Kevin M. Donegan, the commander of the Navy’s Fifth Fleet, suggested that these seizures were part of a larger effort by Iran to move weapons to the Houthis.
“Absolutely it’s not everything,” he said of the four seizures in 2015 and 2016. “These are the ones that I know of because we were able to interdict them.”
Admiral Donegan noted, however, that the captains operating the vessels are typically “out-of-work fishermen, smugglers; they’re not necessarily working for the government” of Iran. He added that the evidence of Iran’s hand in the shipments, while strong, was not ironclad.
This echoed the report by Conflict Armament Research, which said that antitank weapons apparently seized in Yemen have matched lot numbers for the same class of weapons seized on Iranian dhows but stopped short of claiming to have clear proof of an Iranian government hand.
The consultancy also documented weapons manufactured by China, Russia, Romania, Bulgaria and perhaps in North Korea in seizures from the dhows.
The consultancy also did not suggest that the evidence indicated a direct handoff of weapons from the dhows to Houthi forces. Rather, it said, the weapons appear to be offloaded in Somalia and transferred to smaller vessels for smuggling into southern Yemen.
Weapons from Iranian dhows would not be alone in reaching the conflict, which has been fueled in part by extensive arms transfers by outside governments.
Western governments, including those of the United States, Britain and Canada, have provided billions of dollars worth of weapons and military equipment, as well as intelligence and logistics support, to the Saudi-led coalition, which has been waging an extensive bombing campaign against the Houthis.
Among the American-provided weapons have been GBU-series guided bombs and cluster munitions, both of which have been linked by human rights groups and journalists to attacks on Yemeni factories and civilian deaths.