BAGHDAD (AP) — Iraq’s military said five security force members and a civilian were killed early Friday in a barrage of U.S. airstrikes launched hours after a rocket attack killed and wounded American and British servicemen at a base north of Baghdad.

An Iran-backed Shiite militia group vowed to exact revenge for the U.S. strikes, potentially signalling another cycle of tit-for-tat violence between Washington and Tehran that could play out inside Iraq.

U.S. officials said the airstrikes’ intended targets were mainly weapons facilities belonging to Kataib Hezbollah, the militia group believed to be responsible for Wednesday’s attack on Camp Taji base.

However, Iraq’s military said the strikes killed three Iraqi army commandos and two federal police officers. A Shiite endowment in the holy city of Karbala said the strikes resulted in the death of one civilian in an airport complex being constructed by the endowment.

A gaping crater was left in the wake of the U.S. airstrike outside Karbala’s defunct airport. Iraqi forces inspected an area covered in shards of glass and debris.

Kataib Hezbollah did not immediately comment on the airstrikes. But Harakat Hezbollah al-Nujaba, an Iran-backed militia

group designated as a terrorist organization by the U.S., vowed to retaliate with “an eye for an eye” in a statement Friday.

The Iraqi military statement also said the U.S. strikes wounded five militia members belonging to the Popular Mobilization Units, an umbrella group that comprises an array of Iraqi militia groups, some of them Iran-backed.

Wednesday’s attack on Camp Taji was the deadliest to target U.S. troops in Iraq since a late December rocket attack on an Iraqi base that killed a U.S. contractor and set in motion a series of tit-for-tat attacks that brought Iraq to the brink of war.

American airstrikes targeting the Iran-backed militia Kataib Hezbollah led to protests at the U.S. Embassy in Baghdad.

A U.S. drone strike in Baghdad then killed Iranian Gen. Qassem Soleimani, a top commander responsible for expeditionary operations across the wider Mideast. Iran struck back with a ballistic missile attack on U.S. forces in Iraq, the Islamic Republic’s most direct assault on America since the 1979 seizing of the U.S. Embassy in Tehran.

The U.S. and Iran stepped stepped back from further attacks after the Soleimani incident. A senior U.S. official said in late January, when U.S.-Iran tensions had cooled, that the killing of Americans constituted a red line that could spark more violence.

Iraq’s military and senior officials condemned Friday’s airstrikes, saying they were in breach of the country’s sovereignty, potentially reversing an ease in U.S.-Iraq diplomatic tensions that reached fever pitch following the assassination of Soleimani on Iraqi soil.

“The pretext that this attack came as a response to the aggressive action targeting Taji Camp is a false one, and leads to escalation without providing any solutions to control the situation,” the statement said.

Iraq’s caretaker Prime Minister Adel Abdul-Mahdi has already ordered an investigation into the rocket attack on Camp Taji, calling it “a very serious security challenge and hostile act.”

Iraq’s presidency said the continual violations of sovereignty constituted a “serious weakening” of the state’s capabilities during a time when the country is facing unprecedented challenges politically, economically and by the coronavirus pandemic.

The statement called on the international community to support Iraq in respecting its sovereignty.

Senior political leaders Ammar al-Hakim of the Hikma Movement and Muqtada al-Sadr, head of the Saeroon bloc, warned against turning Iraq into an arena for conflict and called for restraint.

In Baghdad’s Tahrir Square, the hub of a six-month protest movement, crowds gathered to reject the U.S. strike.

“We condemn the attacks on the Hashed al-Shaabi (Popular Mobilization Forces) facilities in Iraq,” said Ali Muften, a protester. “America and Iran threaten each other but ultimately Iraqis are the victims.”