The Navy disclosed Monday that it recently confiscated a weapons cache from a small fishing craft in the Arabian Sea, seizing about 1,500 Kalashnikov rifles, 200 rocket-propelled grenade launchers and 21 .50-caliber machine guns. It marks the fourth seizure by a U.S.-led maritime task force in the region since September — and underscores the difficulties the United States faces in stopping weapons smuggling to nations like Yemen, where Houthi rebels continue to rely on Iranian arms.

The weapons commonly move on a small craft known as a dhow, a traditional sailing vessel in the Middle East. Foreign policy and military experts said the smuggling has occurred for years, but it comes now at a sensitive time in which the Obama administration is trying to manage the nuclear agreement it reached last year with Iran. International economic sanctions against Tehran were lifted this year as part of the deal in exchange for Iran sending the bulk of its enriched uranium out of the country, disabling one nuclear reactor and shelving the majority of its centrifuges.

Iran has continued a variety of other actions in the Middle East that the United States considers destabilizing. While the U.S.-led coalition has confiscated several shipments of weapons, there’s no way of knowing how many boats have made it through to Yemen, said Michael Knights, a fellow with the Washington Institute, a think tank focused on Middle Eastern issues. The U.S. military is “stuck in the middle” as the United States does “this kind of dance between two extremes” inside Iran.

“We’re trying to help the more moderate elements of the [President Hassan] Rouhani government in Iran to justify Iran’s involvement in the nuclear deal by bringing more tangible economic fruits as sanctions end,” Knights said. “But we’re also trying to signal to hard-line Revolutionary Guard elements that they need to cease their destabilizing actions, such as firing missiles in Gulf shipping lanes and providing weapons to U.N.-embargoed Yemen.”

Retired Navy Adm. Jim Stavridis, who led a carrier strike group in the Persian Gulf in the early phases of the Iraq and Afghanistan wars, said the Persian Gulf, Strait of Hormuz and northern Arabian Sea have been hotbeds of smuggling for many years. But their use to supply arms to Yemen is relatively new, and stopping the flow is a “difficult tactical proposition,” he said. There are thousands of dhows at sea every day, and many are used for legitimate shipping and fishing purposes.

Stavridis, now the dean at Tufts University’s Fletcher School for international affairs, said the United States must rely on intelligence from the international coalition that has been built over the last two decades to identify and capture dhows that are carrying weapons. “A very key element in all of this of course is surveillance of cell phone technology,” the retired admiral said. “But overall, the key is international inter-agency and private-public cooperation.”

A Navy spokesman in the region, Cmdr. Kevin Stephens, declined to discuss trends in the confiscation of weapons, saying only it “speaks to the intelligence that led us to these recent successes.” But he said the United States and its partners will continue to carry out maritime operations in the region to disrupt the flow of illicit arms to the Houthis in Yemen.

“These weapons only serve to exacerbate the situation there and prolong the conflict,” Stephens said.

A U.S. Navy officer, speaking on condition of anonymity due to the sensitivity of the interdiction missions, said the most recent boarding of a dhow carrying arms was “likely straightforward.” The crew had no incentive to resist the U.S. troops from the USS Sirocco, a coastal patrol ship, and were likely just trying to make money, said the officer, who has served in Special Operations forces and led boarding teams against smugglers in the past.

But the officer said the rules of engagement are challenging when facing Iranians. A general rule of thumb, he said, is that about 10 to 15 percent of all illicit cargoes are interdicted. They include not only weapons, but drugs and migrants who are smuggled from countries like Syria.

“You’ll notice the vessel and crew were released.  This was likely to avoid perturbing the Iranians, especially given the recent detainment of our own sailors,” the officer speculated. “This vessel and crew will likely be back to smuggling shortly.”

Stephens said the Navy does not have the legal authority to detain interdicted crews. Seizures take place in international waters, and the Navy crews must eventually let dhow crews go, he said.

The interdictions are known as VBSS missions, short for visit, board, search and seizure. Specific training for VBSS was created by the Navy in the 1990s following the Gulf War as a way of standardizing maritime interception operations that were introduced in the Persian Gulf as part of U.N. resolutions, according to the Navy.

The first level — likely used in the seizure last week — focuses on ships that comply with the instructions of an inspection team, while the second focuses on crews that do not. The third level of VBSS calls for U.S. troops to board a non-compliant vessel whose main deck is 25 feet or less above the water, and the fourth calls for Special Operations troops to raid a larger vessel whose main deck is more than 25 above water, sometimes through the use of helicopters.

Navy SEALs often carry out high-end VBSS missions. But they also can be taken on by elite Marine Corps units, including Raider and Force Reconnaissance teams.

Update: This story was updated to clarify what the levels of VBSS entail.