For more than a year, Houthi rebels in Yemen attacked merchant vessels and warships in the Red Sea with missiles, drones and speedboats loaded with explosives, disrupting global trade through one of the world’s busiest shipping lanes.
Claiming solidarity with Palestinians in Gaza, the Houthis have hit vessels as far as 100 miles off the Yemeni coast, prompting retaliatory airstrikes by U.S. and Israeli warplanes.
The Houthis, who are backed by Iran, largely discontinued their attacks when Israel and Hamas reached a cease-fire in January. But evidence examined by weapons researchers shows that the rebels may have acquired new technology that makes drones more difficult to detect and helps them fly even farther.
“It could potentially give the Houthis an element of surprise against U.S. or Israeli military forces if they were to restart any of these conflicts,” said Taimur Khan, an investigator with Conflict Armament Research, a British group that identifies and tracks weapons and ammunition used in wars around the world.
Mr. Khan traveled to southwestern Yemen in November to document parts of a hydrogen fuel cell system that government forces found in a small boat offshore, alongside other weapons known to be used by Houthi fighters.
Hydrogen fuel cells produce electricity through a reaction of oxygen in the air and compressed hydrogen across a series of charged metal plates. They release water vapor but little heat or noise.
Houthi drones powered by traditional methods like gas-burning engines or lithium batteries can fly about 750 miles. But hydrogen fuel cells would enable them to fly three times that distance and make it far more difficult for acoustic and infrared sensors to detect them.
Conflict Armament Research detailed its findings in a report released on Thursday. The group examined shipping documents showing that the fuel cell components were made by companies in China that advertise their use for drones, and compressed hydrogen tanks mislabeled as oxygen cylinders.
It is not yet possible to know if the items came directly from China, Mr. Khan said. But a new source for weapons components could give the Houthis a strategic boost.
Houthi weapons shipments intercepted at sea have typically been made in, or sent from, Iran, Mr. Khan said. “If the Houthis acquired these items on their own, the cargo we saw would suggest a new supply chain from commercial markets that increases their self-sufficiency, instead of just relying on their backers in the region,” he added.
The boat that Mr. Khan inspected was intercepted at sea in August by Yemen’s National Resistance Forces, which is aligned with the country’s internationally recognized government. The items found aboard included guided artillery rockets, small engines manufactured in Europe that can power cruise missiles, radars and ship-tracking devices, as well as hundreds of commercial drones in addition to the hydrogen fuel cell parts.
Hydrogen-based electrical power with fuel cells is decades old, and was used by NASA during the Apollo missions. Its use to power military drones emerged in the late 2000s during the U.S. wars in Iraq and Afghanistan.
In the years since, hydrogen power became more common for military drones. Its ability to extend their range made it attractive for commercial uses, like inspecting pipelines, power lines and offshore wind farms, according to Andy Kelly of Intelligent Energy, a British company that makes hydrogen fuel cells used in drones that several U.S. companies now sell to the Defense Department.
“The longer they can stay in the air, the more data they can collect,” Mr. Kelly said. “They are key for long-range reconnaissance.”
The hydrogen systems can store three times more energy than lithium batteries of an equal weight, he said, allowing the drone operator to carry more weight over a longer distance.
Fuel cells also produce few vibrations to jostle surveillance cameras and other sensors on a surveillance drone, Mr. Kelly said, adding that they can be reused many more times than the rechargeable batteries commonly used to propel drones.
Conflict Armament Research declined to name the Chinese companies that made the components recovered near Yemen, a policy that ensures its researchers can work privately with firms to determine how their products ended up in the hands of various entities.
