BAGHDAD — U.S. military officials in Iraq have grown increasingly alarmed over attacks by Iran-backed militias using drones to evade detection systems around military bases and diplomatic facilities.

In place of rockets, militiamen have turned at times to small, fixed-wing drones that fly too low to be picked up by defensive systems, military officials and diplomats say. An official with the U.S.-led coalition described the evolving drone threat as the military mission’s biggest concern in Iraq.

In April, a drone strike targeted a CIA hangar inside the airport complex in the northern city of Irbil, according to officials familiar with the matter. The drone’s flight was tracked to within 10 miles of the site, but its path was then lost as it moved into a civilian flight path, the coalition official said. The drone’s remains were partially recovered, and preliminary analysis suggested it was made in Iran, a coalition official said. The attack deeply concerned White House and Pentagon officials because of the covert nature of the facility and the sophistication of the strike.

Although no one was harmed in the strike, it prompted a long night of deliberations over how to respond, according to Western officials. Some U.S. officials advocated serious consideration of a military response, including the National Security Council’s coordinator for the Middle East and North Africa, Brett McGurk, said two people familiar with the matter. The Biden administration ultimately decided against taking military action.

A similar drone attack in May on the sprawling Ain al-Asad air base raised similar concerns among coalition commanders about how militias are adapting their tactics, according to officials and personnel on base.

“The damage wasn’t huge but the coalition were very upset. They told our commanders that it was a major escalation,” said one Iraqi soldier stationed at Ain al-Asad, speaking on the condition of anonymity due to the sensitivity of the situation. Ain al-Asad was previously targeted by Iran with ballistic missiles in January 2020 in response to the U.S. assassination of Iranian commander Maj. Gen. Qasem Soleimani earlier that month.

Rocket attacks by Iran-backed groups have at times killed American servicemen and Iraqi security personnel and civilians, prompting retaliatory military action from the United States and pushing Washington and Tehran to the brink of outright conflict on Iraqi soil.

Image without a caption

Although tensions have cooled since President Biden took office, officials worry that future attacks still risk sparking a new cycle of tit-for-tat violence as Iran-backed groups try to push a rump coalition force out of Iraq altogether.

Iraqi pressure on U.S. and other coalition forces to withdraw surged last year amid outrage here over the Trump administration’s decision to kill Soleimani and Iraqi militia leader Abu Mahdi al-Muhandis using a drone strike.

U.S. troop levels have fallen roughly by half since then, in part because Iraqi security forces are now taking the lead rolling up what remains of the Islamic State. But American numbers have also declined in response to increasing rocket attacks, which left some commanders describing their soldiers as sitting ducks. There are now about 3,000 coalition troops in Iraq, including 2,500 Americans.

In the absence of effective defenses, the drone threat now raises the prospect of a sudden escalation of violence. Each fresh attack triggers a flurry of communication as U.S. officials seek to determine whether Americans have been killed or injured.

“The death of an American is their red line,” said one Western official, speaking on the condition of anonymity due to the sensitivity of the issue. “The first question the Americans always ask is: what was the casualty’s nationality?”

The top U.S. military commander in the Middle East, Marine Gen. Frank McKenzie, said last week that efforts were underway to develop better defenses against the drones. While visiting northeastern Syria, McKenzie told reporters that military officials were looking for ways to cut command-and-control links between a drone and its operator, improve radar sensors to quickly identify the threat as it approached, and find effective ways to bring the aircraft down.

“We’re open to all kinds of things,” he said, according to the Associated Press. “Still, I don’t think we’re where we want to be.”

Another official said that electronic jammers are being considered, as are other systems being developed by U.S. Special Forces in Syria.

In April, the White House announced the formation of a joint working group between the United States and Israel “on the growing threat of Unmanned Aerial Vehicles [UAVs] and Precision Guided Missiles produced by Iran and provided to its proxies in the Middle East Region.”

Iraqi security personnel, speaking in interviews, described unease and frustration on military bases because of drone and rocket attacks, and emphasized that the strikes on U.S.-linked facilities pose a threat to Iraqis as well.

“I feel so frustrated when I see these attacks,” said the officer at Ain al-Asad. “We know where they come from, but we can’t do anything about it, even as officers in the Iraqi army.”

The future of the U.S.-led coalition in Iraq is the focus of ongoing discussions between U.S. and Iraqi officials. Although U.S. officials have said that the American-Iraqi relationship is in a new honeymoon period following Biden’s inauguration, Iraqi military officials have voiced frustration that they feel like a junior partner in a relationship largely centered on reducing Iranian influence in the region.

The first troop talks of Biden’s presidency took place in April and although both sides called them a successful step toward reducing the coalition presence in Iraq, the resulting communique appeared to be more a restatement of current realities than a strategic shift.

In a statement this month, a council of Iran-linked militia groups described the latest talks as “totally and completely unacceptable” and vowed to increase pressure on coalition forces. “The Iraqi resistance confirms its full readiness to perform its legitimate, national and legal duty to achieve this goal,” the council said.

Last year, U.S. officials hailed the killings of Soleimani and Muhandis as a way of weakening the threat that Iran-linked groups pose to American forces in Iraq.

Instead, that strike made the threat more diffuse. Iran-linked groups have since rallied, security officials and analysts say, seeding their militia members among a mushrooming number of smaller front groups, which now regularly attack U.S.-linked targets.

Iraq’s militia network, known as the Popular Mobilization Forces (PMF), includes some groups that are backed by Iran and others that are not. They are an official part of Iraq’s security forces, and their members command extensive influence throughout the country’s Iraqi economy and political system. Human rights groups also say the militias are behind a rising tide of assassinations targeting their critics.

In private, Iraqi government and security officials say that they fear the repercussions of any sustained effort to rein in Iran-linked groups that have been launching attacks on Western military targets or shooting activists in the street.

In the early hours of Wednesday morning, Iraqi Prime Minister Mustafa al-Kadhimi tried anyway, ordering the arrest of a senior PMF commander, Qasem Muhsen, who was linked to both. Within hours of the announcement, footage circulated on social media of militiamen insulting Kadhimi as they walked through Baghdad’s heavily fortified Green Zone toward his house.

“Hey Kadhimi, are you asleep, or are you awake?” said one.