The airstrikes were a show of force but appeared calibrated to head off a wider escalation, targeting Iran’s network of proxies but doing little if any direct damage to Tehran’s assets in the region.

“It looks like a very significant action by the Biden administration, but on the other hand I don’t think it’s going to be anywhere near sufficient to deter these groups,” said Charles Lister, director of the Middle East Institute’s Syria program.

“These militias have been engaged in this campaign for more than 20 years, they are in a long-term struggle,” he said. “They are ultimately engaged in an attritional campaign against the U.S.”

The strikes late Friday on 85 targets, carried out using B-1 bombers flown from the United States, were part of what U.S. officials say would be a multiday campaign at regional targets linked to Iran.

The United States, Britain and six other nations also carried out strikes against Iran-backed Houthi militants in Yemen late on Saturday local time. The operation targeted weapons facilities and other military sites, a joint statement said, and was launched in response to repeated Houthi attacks on commercial and naval vessels transiting the Red Sea.

The Houthis, like the militias in Iraq and Syria, have portrayed their attacks as acts of protest against Israel’s punishing war in the Gaza Strip. The Biden administration has sought to contain spillover from that conflict, while continuing to offer unwavering support to Israel. Now, the United States is engaged in near-daily military confrontations with Iran-backed proxies.

On Saturday, Iranian Foreign Ministry spokesman Nasser Kanaani described the strikes in Iraq and Syria as another American “strategic mistake” alongside its support of Israel during its war on Hamas. They contribute to “tension and instability” in the region, he said.

The Syrian Observatory for Human Rights, a U.K.-based monitoring network, said 18 militants were killed in strikes at 26 Iran-linked sites in Syria. The Washington Post could not independently verify the figures.

Iraq, a strategic ally of the United States, said it had summoned David Burger, the U.S. charge d’affaires in Baghdad, to hand him a letter of protest following the raids that killed at least 16 Iraqis and injured 25 in the Akashat and al-Qaim areas of the western province of Anbar.

An Iraqi government spokesman described the American airstrikes as “blatant aggression.”

“This aggressive strike places the security in Iraq and the region on the edge of the abyss, conflicting with efforts to establish the required stability,” spokesman Basim al-Awadi said. He said the government rejected the use of Iraqi soil as a “battleground for settling scores.”

While the Iraqi government said that the 16 deaths included an unspecified number of civilians, Iraq’s Popular Mobilization Forces (PMF), which includes an array of militia factions including some linked to Iran, attributed the same number of casualties — 16 deaths and 25 injuries — to its fighters. It released a list of their names.

Separately, an Iraqi official in Anbar province, who declined to be named due to the sensitivity of the subject, said at least two civilians were killed near al-Qaim on the Syrian border, where weapons depots were targeted, putting the potential number of deaths in the country at 18. Shiekh Hani Al-Awad, a tribal leader from the area, also confirmed the death of two civilians.

Iraqi officials did not respond to queries on the discrepancies.

The PMF is made up of largely Shiite militias that rallied to fight Islamic State militants in 2014 after they overran swathes of Iraq and Syria. The United States also sent troops to support ground and air operations against the Islamic State, and still has a few thousand troops stationed in Iraq and Syria, with a stated mission of preventing the group’s resurgence.

But while they were briefly united in a shared goal, the more hardline Iran-backed proxies soon turned their efforts toward pushing U.S. forces from the region.

The PMF has since been absorbed by Iraq’s formal security forces, while pro-Iran militias coalesced under the Islamic Resistance in Iraq, an umbrella group. The Islamic Resistance of Iraq claimed responsibility for the lethal drone attack against U.S. troops in Jordan on Jan. 28.

The group also said on Saturday that it had launched two new attacks on U.S. troop positions in response to the airstrikes, including one at the Harir airbase in northern Iraq and a second on a base in Kharab al-Jeer in northeastern Syria.

A spokesperson for Operation Inherent Resolve, the U.S. military coalition supporting Iraq’s fight against Islamic State militants, did not respond to requests for comment.

“The fact that they’ve been bold enough to pop their heads up above the parapet today and launch attacks at U.S. troops speaks volumes about their confidence,” Lister said of the Islamic Resistance’s claims on Saturday.

“Now the ball is in America’s court again,” he said. “Is the second round more aggressive? Does the second round include an attempt to target a high-value Iranian-linked operative as another way of trying to send a deterrent message? Or is it just more of the same?”

Proxy groups would be happy to return to “chipping away at American credibility and deployments,” he said.

Abdolrasool Divsallar, an Iran specialist and professor of Middle Eastern Studies at the Università Cattolica del Sacro Cuore in Milan said that the Biden administration appeared to strike a balance between deterrence and deescalation. Iran had nearly a week to prepare for the U.S. response.

“They’ve been evacuating their personnel and assets,” Divsallar said, adding that Iran knows further retaliation could cause an “escalation loop.” Both Washington and Tehran have signaled they want to avoid further conflict, he added, saying he expected the response from proxy groups on the ground to remain “low intensity.”

U.S. Central Command said on Friday that more than 125 precision munitions were fired at assets belonging to “militia groups and their [Iranian military] sponsors who facilitated attacks against U.S. and Coalition forces” during the attack.

Iranian proxies in the Middle East have escalated attacks on the United States and Israel since the start of the war in Gaza. Israel has responded with a military campaign that has killed more than 27,000 Palestinians, according to the Gaza Health Ministry.

The deaths of Spec. Breonna A. Moffett, 23, Sgt. William J. Rivers, 46, and Spec. Kennedy L. Sanders, 24 were the first in what the Institute for the Study of War says has been more than 170 attacks on U.S. military bases in the region, mainly in Syria and Iraq, since Oct. 7. More than 50 troops were wounded, at least one critically, in the strike on Tower 22, a key support base for the larger U.S. installation at Tanf in Syria.

The United States has launched dozens of retaliatory strikes since Oct. 7, including one in Baghdad that killed a senior commander of al-Nujaba.

Washington has also bombed Iranian-linked Houthis in Yemen, who have been attacking commercial shipping in what they say is a protest against the Gaza war.

Salim reported from Baghdad; El Chamaa reported from Beirut. Susannah George in Dubai and Missy Ryan, Dan Lamothe and Alex Horton in Washington contributed to this report.