DUBAI—Iranian support has helped the Houthis mount the most potent and sophisticated attacks yet on Saudi Arabia and its Yemeni allies, including help targeting drone and missile attacks, as well as training, intelligence and tactical guidance, people briefed on intelligence from Saudi Arabia and Yemen’s anti-Houthi alliance said.

The assistance has contributed to a Houthi offensive that has produced rapid territorial gains in Yemen. The militant group now controls the coastline on its side of the Bab al-Mandeb Strait, a chokepoint for Saudi oil exports, while it has launched attacks on Saudi oil tankers, energy facilities and airports, including a strike that killed at least 13 people last weekend in Riyadh.

Iran’s contributions highlight its ability to use its integration with Middle East militias like the Houthis to ratchet up pressure in the region—a challenge as President Trump and America’s Gulf allies try to wind down the war with Iran on favorable terms.

“The connection is very, very strong on the operational level,” said Danny Citrinowicz, a former head of the Iran branch for Israeli defense intelligence. “Iran has a major interest in the Houthis’ success. It harms Saudi Arabia, it raises the oil prices. Therefore there is a shared strategic interest that isn’t based on a client-patron relationship, but rather on strategic cooperation.”Saudi Arabia has been fighting the Houthis on and off for more than a decade. The group surged out of the mountains in northern Yemen to take the capital, San’a, in 2014, and seized armaments, such as missiles, left behind by the fleeing government’s military. Saudi Arabia and the United Arab Emirates lined up with forces loyal to the government and fought back.

Iran sided with the militant group. It sent arms hidden on traditional dhow boats and brought in operatives from the Islamic Revolutionary Guard Corps and the Lebanese militia Hezbollah to train the Houthis to assemble their own missiles and drones and use them more effectively. When shipping arms by boat got interrupted by U.S. and Israeli strikes earlier this decade, Iran sent weapons to Yemen by submarine, European and Houthi officials said at the time.

The latest outbreak of fighting shattered a four-year truce and dates back to July, when Iran sent two flights to Houthi-controlled territory to ferry Yemeni pilgrims and officials with the militant group to and from Tehran for the funeral of former Supreme Leader Ali Khamenei. The flights were on Mahan Air, an Iranian airline under sanctions for ferrying cash, gold, weapons and personnel for the regime’s Revolutionary Guard.

The flights were carrying something else, too: Iranian paramilitary officers and a cargo hold full of weaponry including drone parts, missile guidance systems and impact detonators, according to people familiar with intelligence from Saudi Arabia and Yemen’s anti-Houthi alliance.

Discovery of the secret cargoes led Saudi-aligned forces to try to stop the second flight by bombing the runway of the Yemeni airport in San’a—lighting the fuse on the new conflict.

Mohammed al-Bukhaiti, a senior Houthi official and spokesman who said he was on the second flight, called the allegations about Revolutionary Guard officers and weapons an exaggeration to justify the Saudi attack.

“Portraying this civilian flight as an operation to transport military commanders and weapons is nothing more than an attempt to exaggerate the incident, justify the attack on San’a airport and deny citizens their right to travel,” he said.

The militants began building up positions threatening the Red Sea and Bab al-Mandeb in several locations, deploying missiles and radars with help from Revolutionary Guard officers inside Yemen, people briefed on regional intelligence said.

When the Houthi offensive got rolling in early September, the militants deployed large numbers of fighters backed by missiles and drones in a well-planned attack. Yemeni forces aligned with the internationally recognized government and their Saudi backers were caught off guard as the militants swept into the Red Sea port city of Mokha and took territory in and around the Bab al-Mandeb.

The Houthis’ tactics showed more sophistication than in previous fights, combining infantry assaults with attacks by missiles and drones, including first-person-view drones guided by fiber-optic wire, that pointed to training by Iran, a person with the forces opposing the militants said. Revolutionary Guard officers were on the front lines helping direct drone and missile attacks, another anti-Houthi official said.

The combined impact of the Houthi gains along the Red Sea and Iran’s threat to traffic in the Strait of Hormuz has expanded the threat to U.S. and allied interests, raising the costs of the war.

“It is helping Iran project power,” the Soufan Center, a New York-based security think tank, said of the Houthis’ gains.

The Houthis are fiercely independent. They make some of their own weapons, and hew to domestic priorities that, until the summer, kept them on the sidelines of the war between the U.S. and Iran. Researchers have found the Houthis now source parts for drones and missiles directly from China.

Bukhaiti, the senior Houthi official and spokesman, said the claims of Iranian help ignore the group’s own progress in developing its capabilities and are a baseless attempt to explain away the Saudi-backed forces’ losses.

“Iran does not interfere in Yemeni affairs,” Iranian Foreign Ministry spokesman Esmail Baghaei told reporters last month, saying the Houthis are “entirely independent actors who make their decisions based on their own interests.”But the Houthis also operate as part of a network of Iran-backed regional militias that share know-how, weaponry, personnel and goals. The Houthis have set up joint operations rooms in Lebanon and Iraq to coordinate activities, a United Nations panel of experts found in 2024. During the fiercest fighting of the war with Iran, as many as half the attacks on Saudi Arabia came from inside Iraq.

Iran built up the militias into a self-described Axis of Resistance to project power and keep fighting away from its borders. Revolutionary Guard officers are assigned to support units involved in every aspect of Houthi military capability—drones, missiles, tunnels, logistics—people familiar with the Saudi intelligence said.

Members of the Revolutionary Guard recently praised the Houthis’ battlefield advances and air attacks on social media.

“The support the Houthis received from Iran and other Axis actors undeniably paved the way for the rapid-fire gains they’ve made over the past few months,” said Adam Baron, a Yemen expert and fellow with New America, a policy institute in Washington.

Write to Summer Said at summer.said@wsj.com and Benoit Faucon at benoit.faucon@wsj.com