RIYADH, Saudi Arabia—Iraqi militias backed by Iran launched dozens of explosive drones at Saudi Arabia and other Gulf states during more than five weeks of fighting, in what is becoming a shadowy war within a war pushing some of the world’s largest oil producers toward open conflict.
According to at least one Saudi assessment described by a person familiar with it, up to half of the nearly 1,000 drone attacks on the kingdom came from inside Iraq. They included strikes on a Saudi refinery in the sensitive Yanbu oil hub on the Red Sea and oil fields in the kingdom’s Eastern Province, people familiar with the matter said.
Drones launched from Iraq targeted Kuwait’s only civilian airport. They also targeted Bahrain after President Trump announced a cease-fire earlier this month, some of the people said. Militias went after Gulf assets inside Iraq as well, including the Kuwaiti consulate in Basra and the United Arab Emirates’s consulate in Kurdistan.
The conflict is unfolding in the shadow of the war the U.S. and Israel launched against Iran in late February. Iran itself has fired thousands of drones and missiles at its Gulf Arab neighbors, as well as at Israel and U.S. bases across the region.
The militias in Iraq—along with Iranian-backed Hezbollah in Lebanon, which fired rockets at Israel throughout the war—broadened Iran’s options for attacking its enemies and the amount of firepower it could rain down.
The U.S. has warned the militias are planning more attacks and has told citizens to stay away from the embassy and consulates in Iraq. The American Embassy in Baghdad has been targeted repeatedly throughout the war and has largely evacuated its staff.
Iraq’s Shia militias grew out of the chaos after the U.S. invasion more than two decades ago. They defended Shia areas against attacks by Sunni militants and fought American forces their leaders denounced as occupiers. Iran funneled arms to many of the groups, which later took on a major role in fighting Islamic State fighters who swept into Iraq from Syria in 2014.
There are now dozens of militias with around a quarter million members, several billion dollars of funds and an arsenal that includes long-range missiles. The most potent—Kataib Hezbollah and Asaib Ahl al-Haq—hold considerable clout with both the Iraqi and Iranian governments.
They have long threatened Saudi Arabia, the U.A.E. and Kuwait over their opposition to Iran and ties to the U.S. They have landed a few hits—including in 2021, when one fired drones at the main royal complex in the Saudi capital Riyadh and a desert encampment used by the royal family, which was away at the time.
Neither the Iraqi militias nor Hezbollah played a notable role in the June war with Iran last year. What is different now is that the regime faces an existential threat, endangering the militias as well. They are responding by acting with less restraint and in some cases are working directly within the Iranian military command structure, analysts said.
Gen. Esmail Qaani, a top officer with Iran’s Islamic Revolutionary Guard Corps responsible for cultivation of overseas militias, visited Baghdad over the weekend.
The Gulf states, after weeks of punishing Iranian strikes that have included some precise hits on energy infrastructure, see Iraq as a place where they can respond without directly attacking Iranian territory and provoking even harsher reprisals.
“Iraq is the place where they can all hit back, and it’s fair game,” said Michael Knights, head of research for strategic advisory firm Horizon Engage and adjunct fellow at the Washington Institute think tank. “If they need to show that nobody’s getting any free hits on them, that’s a good place to flex.”
Knights, who has studied the Saudi-led military campaign against Iran’s Houthi allies during Yemen’s civil war, said Saudi Arabia was likely to start launching symbolic strikes in Iraq as a warning to the militias, while Kuwait and Bahrain could allow the U.S. to use their territory for missile attacks on Iraqi militias.
The militia attacks have strained ties between Iraq’s government and its Gulf neighbors, said Abdel Aziz Aluwaisheg, assistant secretary-general for political and negotiation affairs at the Gulf Cooperation Council, an intergovernmental organization for the six Gulf monarchies.
In some instances the militias are showing themselves to be stronger than the national government, much like Hezbollah in Lebanon—a problem for Gulf states that are trying to build a relationship, he said.
“The Iraqi government needs to exercise control,” Aluwaisheg said.
Iraq has a troubled history with its neighbors. Dictator Saddam Hussein invaded Kuwait in 1990 and threatened to roll into Saudi Arabia, seeking control of nearly a third of the world’s proven oil reserves.
Hundreds of thousands of U.S. troops flooded into Saudi Arabia and Kuwait to repel the invasion, prompting Iraq to launch dozens of Scud missiles at Saudi Arabia. U.S. forces returned in 2003 to topple Hussein’s regime, with Kuwait the primary staging ground for the invasion.
The Iran war has coincided with a period of political turmoil in Iraq following parliamentary elections in November. Infighting over who should form the new government has handicapped Iraqi leaders’ ability to disarm militias and purge them from the state apparatus.
Former Prime Minister Nouri al-Maliki, who has close ties to the militias and Iran, emerged as front-runner for the top job before Trump objected early this year.
The emboldened militias are less restrained than they were just a few years ago, and the Revolutionary Guard is more involved in supporting their attacks, said Renad Mansour, director of the Iraq Initiative project at Chatham House, a London think tank.
“The prospect of regime collapse or fragmentation or degradation even in Tehran, especially for these resistance groups, is existential, because this is their main source of power,” Mansour said. “The strategy for Iran and therefore these groups which are under Iran’s command in this war is to spoil, is to disrupt and to show the consequences” of the war against Iran.
Write to Stephen Kalin at stephen.kalin@wsj.com