DUBAI — Despite months of U.S.-led airstrikes against Yemen’s Houthi fighters, the once ragtag rebels have continued to threaten some of the world’s most vital shipping routes, drawing from an arsenal of increasingly advanced weapons to attack vessels in and around the Red Sea.

Just this month, Houthi militants sank one ship and set another ablaze. The fighters, operating on land and in the water, have launched swarms of drones at U.S. warships and deployed a remote-controlled boat packed with explosives, tactics and weapons that experts say are associated with the group’s patron, Iran.

The recent uptick in Houthi activity has underscored the group’s ability to pose a sustained threat, relying in part on a steady flow of Iranian arms and expertise both to withstand U.S. strikes and remain on the attack. The faltering U.S. efforts to halt Houthi operations and protect global shipping have also drawn scrutiny from Congress, where lawmakers say not enough is being done to establish deterrence.

“Their ability to replace whatever we destroy is unimpeded and our ability to interdict materiel coming into the country negligible,” said Gerald Feierstein, a former U.S. ambassador to Yemen who is now a senior fellow at the Middle East Institute in Washington.

For years, Iran has circumvented a United Nations arms embargo against Yemen, covertly sending weapons and equipment from Iranian ports to the Arabian Sea, or overland from neighboring Oman. The Houthis have also learned how to modify old weapons and manufacture new ones, becoming the first group to use anti-ship ballistic missiles to strike naval targets, according to senior U.S. military commanders.

“Their capability has definitely increased” since they started their campaign, Feierstein said. “So as long as they have the incentive to continue these attacks, they’ve demonstrated they have the ability to do it.”

The Houthi movement, whose leaders represent a minority Shiite sect in northern Yemen, first emerged in the 1990s and later seized the capital, Sanaa, amid the chaos of the Arab Spring in 2014. They fought a bruising war with Saudi Arabia, which wanted to eliminate an Iranian proxy on its border, but ultimately stayed in power and expanded the amount of territory they control.

Experts estimate that the group has a fighting force of at least 20,000, including a mix of tribal forces and troops formerly loyal to the government.

In November, after war broke out between Israel and Hamas, the Houthis announced they would begin attacking Israeli-linked ships in solidarity with Palestinians in Gaza. Their first major salvo included hijacking a cargo vessel in the southern Red Sea and detaining its crew.

Since then, the Pentagon has recorded more than 190 attacks on either U.S. military vessels or commercial shipping off the coast of Yemen, including nearly 100 since waves of U.S. airstrikes began in January.

The Houthis have sunk two ships, including the Rubymar in March and the Greek-owned Tutor coal carrier that was hit in the stern last week by an explosives-filled surface vessel. Also in March, an anti-ship ballistic missile fired by the Houthis set the Barbados-flagged True Confidence on fire, killing three people.

The operations soon broadened to the Gulf of Aden and Bab el-Mandeb Strait, which connects the Indian Ocean to the Red Sea. From there, ships transit through the Suez Canal to the Mediterranean, the shortest maritime route between Europe and Asia.

But the security threats have cratered Red Sea shipping traffic, and by the end of March, the volume of traffic through the Suez Canal and Bab el-Mandeb Strait had dropped by half, according to the World Bank.

The Houthis “will continue to understand that there’s a price to be paid” for harming maritime trade in the region, a Pentagon spokesman, Maj. Gen. Patrick Ryder, told reporters Tuesday, calling the attacks “unacceptable.”

The Pentagon has deployed a rotating cast of warships in the region in an effort to thwart the Houthi threat, shooting down drones over the Red Sea and other waterways and striking missiles and radar sites in Yemen.

The effort includes an aircraft carrier, the USS Dwight D. Eisenhower, and the destroyers and other warships deployed with it. The Eisenhower deployed in October and has seen its mission extended twice by Defense Secretary Lloyd Austin, as the Pentagon prioritizes keeping firepower in the region.

But Republican lawmakers, some of whom are pushing for a dramatic surge in Pentagon spending in the year ahead, have accused the Biden administration of underinvesting in the advanced weapons and surveillance technology they now say are necessary for the fight.

“We just simply don’t have the political will to go after them,” Sen. Mike Rounds (R-S.D.), who sits on the Senate intelligence and armed services committees, said in an interview Tuesday.

He attributed the rise in Houthi attacks to “resources that are being directed to them by Iran,” as well as “enhanced technology that has made their systems more accurate.”

“Each of the different system types has their own capability,” said Rounds, who declined to comment on specific weaponry. “I don’t want to get into what it is that’s the most significant, but it’s more advanced than what they had to begin with,” he said.

The administration in March said that it was expanding efforts to intercept Iranian weapons being smuggled to Yemen. And on Monday, the Treasury Department’s Office of Foreign Assets Control said it was targeting with sanctions several individuals and entities involved in weapons procurement for the Houthis.

Sen. Mark Kelly (D-Ariz.) said that the U.S. naval destroyers and aircraft carrier battle groups in the region have been “rather successful” at disrupting attacks. U.S. forces, he said, have “expended a lot of munitions in order to protect shipping.”

“But if we don’t protect that shipping, we’re going to see increased supply chain problems,” Kelly, a Navy veteran who serves on the Senate Armed Services Committee, said in an interview Tuesday.

He said he had just reviewed classified intelligence on the issue and could not comment in detail on efforts to disrupt Iranian weapons shipments to the Houthis. But he acknowledged that the Houthis were continuing to acquire advanced weaponry from Iran.

“I think as they get munitions from the Iranians that they feel it’s in their best interest to use them in disruption in the Red Sea,” Kelly said.

For the Houthis, the relative success of their Red Sea campaign has given them the flexibility to more easily maneuver in the region and at home.

“This is an attempt to demonstrate that the Houthis are a serious regional actor,” said Hannah Porter, a Yemen researcher with ARK Group, a U.K.-based international development organization. After engaging in direct combat with the U.S. military, Porter said the Houthis “can now portray themselves as a power player,” and use that to tighten their grip domestically or in ongoing peace talks with Saudi Arabia.

On the ground in Yemen, that work is already underway. Images from the conflict - including video of the November hijacking and missile strikes on other vessels - are used by the Houthis to both drive recruiting campaigns and crack down on dissent, according to researchers and local media reports. Houthi media outlets have reported that tens of thousands of additional fighters have joined their ranks since the Red Sea attacks began.

“The Houthis are very good at seizing opportunities to assert themselves,” said Nadwa al-Dawsari, a Yemeni researcher now based in the United States with the Middle East Institute. “And in this case, they are using the Red Sea attacks to gear up for escalation in Yemen.”

Earlier this month, the Houthis launched a widening crackdown abducting aid workers with the United Nations and the Washington-based National Democratic Institute.

Dawsari said the arrests are aimed at extinguishing the small pockets of dissent that remain in Houthi-controlled Yemen. “These voices have been suppressed, but now the Houthis want to eliminate them completely,” she said.

Lamothe and Hauslohner reported from Washington.