The Biden administration is expanding efforts to surveil and intercept Iranian weapons being smuggled to Yemen, where Houthi militants have staged a deadly campaign of violence against commercial shipping that has proved resilient to six weeks of military strikes, said U.S. officials familiar with the matter.

The initiative seeks to map seafaring routes used by Tehran and stop the arms shipments while in transit, an acknowledgment that the Houthis are likely to pose a significant security challenge for the foreseeable future. It is part of a broader strategy that also includes sanctions and diplomatic pressure, but faces constraint as essential military resources are in short supply.

A senior U.S. defense official described the evolving mission as “a renewed effort to try to better understand what those water routes look like.” Like others interviewed for this report, the official spoke on the condition of anonymity to describe sensitive military activity. The work requires considerable collaboration with the U.S. intelligence community, the official said.

A second senior defense official characterized the effort as “very vigorous,” saying Washington also is exploring how partner nations can expand their focus on disrupting Iranian arms smuggling to help offset a limited inventory of U.S. drones and other surveillance assets that are central to the process. The official declined to identify which nations are involved in those conversations, but said all governments affected economically by the Houthi attacks should do more.

“It’s definitely a challenge in an area as large as the one we are describing to identify all of these craft,” this person said. “But we are devoting significant resources to identifying, tracking and - where we have the ability - interdicting. And what we are finding is significant.”

The Houthis, who rose from a ragtag band of rebels to functioning now as the de facto government overseeing much of Yemen, fall under Iran’s regional network of proxy forces opposed to Israel and the U.S. military presence in the Middle East. The group’s leaders have characterized its actions in the Red Sea and the Gulf of Aden as a demonstration of solidarity with Hamas fighters battling Israeli forces in Gaza, yet often its targeting has appeared indiscriminate - it once even fired on a ship hauling grain to Yemen, where conflict has left millions in hunger, according to aid organizations.

When Houthi fighters seized Yemen’s capital, Sanaa, in 2014, they inherited an array of weaponry, including North Korean and Soviet-era scud missiles, Soviet-era surface-to-air missiles, and Chinese anti-ship missiles, said Mohammed al-Basha, a senior Middle East analyst at the Navanti Group. Since then, the group has learned to create more advanced weapons by modifying items in its arsenal and using technology obtained from overseas, including from Iran.

Since November - shortly after the Oct. 7 Hamas assault on Israel that ignited the war in Gaza - the Defense Department has documented at least 105 attacks on merchant vessels off Yemen, including about 40 over the past week. The weapons include one-way attack drones, rockets, ballistic missiles and explosive-laden drones that can skim the waves and travel underwater, officials said.

A U.S.-led effort to protect maritime traffic has successfully thwarted many of those attacks. On March 6, however, an anti-ship missile launched by the Houthis struck a commercial vessel, the MV True Confidence, in the Gulf of Aden. At least three mariners were killed and several more were injured, U.S. officials said. Last month, a Houthi missile strike on the MV Rubymar, a U.S.-owned cargo ship, caused the vessel to sink.

While the United States conducted a campaign against al-Qaeda militants in Yemen for more than a decade, it devoted limited attention to the Houthis, who despite their anti-American rhetoric were more focused on countering an air campaign by Saudi Arabia than attacking U.S. or Western interests. As a consequence, the Pentagon today has a somewhat narrow understanding of the group’s smuggling operations, current and former officials say.

Maritime smuggling has originated from Iranian ports such as Bandar Jask, in the Gulf of Oman, and Bandar Abbas, in the Strait of Hormuz, according to U.N. experts. Such shipments can be transported through the Arabian Sea and the Gulf of Aden all the way to Yemen, or take routes over land through bordering countries such as Oman.

At least 18 maritime interdictions have occurred since 2013, revealing shipments of weapons alleged to have come from Iran ranging from machine guns to antitank missiles, said al-Basha. Additional smuggling has occurred via the Horn of Africa.

It is unknown how much materiel has gotten through undetected, making it difficult for the United States to assess the effectiveness of its recent strikes - there have been dozens dating to January - in degrading the Houthis’ ability to continue their maritime attacks.

A persistent challenge facing the U.S. military is its finite number of drones and other surveillance assets, which are in high demand by American military leaders across the world. The Pentagon, as part of a shifting global security strategy intended to focus foremost on China, in recent years reassigned some of that equipment that had been in the Middle East over two decades of war in Afghanistan and Iraq.

Gen. Michael “Erik” Kurilla, who as head of U.S. Central Command oversees American military activity throughout the Mideast, told the Senate Armed Services Committee this month that “for a time,” he diverted surveillance capabilities from over Afghanistan - where the United States continues to monitor terrorist groups - to focus instead on the Red Sea, as well as Iraq and Syria, where until recently deployed U.S. forces faced repeated attacks from groups aided by Iran.

Kurilla said the United States needs to fund more as “additional capabilities.”

The Houthis have shot down at least two MQ-9 Reaper drones off the coast of Yemen, once in November and again in February, U.S. officials said.

Another limitation is the availability of highly trained personnel available to carry out the perilous task of boarding vessels suspected of carrying Iranian weapons to Yemen. Although the Pentagon is stepping up its interdiction efforts, the mission is not expected to entail a major allocation of additional Special Operations forces, officials said.

Marine Corps forces deployed aboard ships also have historically participated in such missions, but for the foreseeable future, none are expected in the region because of an ongoing shortage of available amphibious ships overseen by the Navy, U.S. officials said. The 26th Marine Expeditionary Unit recently departed the Red Sea region after a lengthy deployment and is expected to arrive home in North Carolina in coming days.

Glimpses of the evolving mission have emerged through the handful of ship-boarding operations disclosed to the public in recent months.

On Jan. 11, two Navy SEALs were lost at sea while attempting to climb aboard a suspected smuggling vessel off Somalia. Others involved, including U.S. Coast Guard members, recovered what Centcom said was an array of Iranian-made weapons, including missile components, and took 14 people into custody. Four of them face charges, including intentionally transporting a warhead, the Justice Department announced in February.

A month later, Coast Guard personnel intercepted a vessel in the Arabian Sea and seized ballistic missile components, explosives and other weapons parts, officials said. The shipment originated in Iran, they said.

Carl “Sam” Mundy III, a retired lieutenant general who oversaw Marine Corps forces in the Middle East from 2018 to 2021, called these missions among the military’s most dangerous and unpredictable. They can happen with U.S. forces “fast-roping” from helicopters down to the suspected smuggling ship or boarding from the water after swooping in on small, high-speed boats.

“Many times, we don’t know what exactly the threat is,” said Mundy, a distinguished senior fellow with the Middle East Institute. “A lot of times, we don’t know. And so, of course, that complicates the operation because you’re putting people in a vulnerable situation and adding in all these atmospheric conditions that make it all very challenging.”

Boarding can be carried out by SEALs, Force Reconnaissance Marines, Coast Guard maritime security response teams and other elite forces. Gathering intelligence and making sense of it are required to make such missions successful, and that takes time, he said, especially in an area as vast as the Red Sea and nearby waterways.

“The problem is, it’s a big geographic area and we don’t have enough resources to do this,” Mundy said. “To do this right, it’s going to take time.”

Kenneth “Frank” McKenzie Jr., a retired Marine Corps general who led Centcom from 2019 to 2022, said cutting off the flow of lethal arms from Iran to the Houthis is critical.

“We need to recognize that, and we need to put resources against it,” McKenzie said. Principally, that requires surveillance resources, he said, but “also the platforms that allow us to actually do the intercepts, and we need to work with our coalition partners in order to do this.”

Elana DeLozier, a Yemen expert who runs the Sage Institute for Foreign Affairs, said it is unclear whether the Houthis will halt their attacks if large-scale Israeli military operations conclude in Gaza. It’s possible, she said, “that the goal posts could move,” considering that the Houthis appear to derive other benefits from taking up the Palestinian cause.

One such benefit is that other Yemeni groups that are typically the Houthis’ adversaries must consider whether they may be portrayed as not sufficiently pro-Palestinian if they attack the Houthis.

“It becomes a black-and-white thing,” DeLozier said, “that is convenient for the Houthis.”