The U.S. military has launched strikes on five facilities in Iraq and Syria belonging to a militia considered to be backed by Iran, the Pentagon said Sunday, two days after an American contractor was killed in an attack.

The strikes came after repeated assaults by the militia, Kitaeb Hezbollah, on Iraqi bases, Jonathan Hoffman, the chief Pentagon spokesman, said in a statement. The American operations will “degrade KH’s ability to conduct future attacks” against coalition forces, he said.

The actions underscore the continued unpredictability of U.S. involvement in Iraq and Syria and raise the possibility of an escalation with the militia. The Pentagon says the group has links to Iran’s Quds Force, a special operations unit that U.S. officials say provides weapons and other support to proxy forces that help Iran extend its reach.

Under President Trump, United States-Iranian relations have taken a decisive turn for the worse. Here's a brief history of the tumultuous relationship. (Joyce Lee/The Washington Post)

“Iran and their KH proxy forces must cease their attacks on U.S. and coalition forces, and respect Iraq’s sovereignty, to prevent additional defensive actions by U.S. forces,” Hoffman said.

U.S. forces struck three targets in Iraq and two in Syria, the Pentagon said. They include weapons storage facilities and locations the militia uses to plan attacks against coalition forces, Hoffman said.

The Iraqi military’s Joint Operations Command said four fighters, including the deputy commander, were killed in U.S. strikes on Kitaeb Hezbollah’s headquarters along the Iraq-Syria border.

Jaafar al-Hussaini, a spokesman for the militia, said 19 fighters were killed and 35 were injured in the strikes. “For those who ask about the response: it will be the size of our faith,” he said in a statement.

Pentagon officials have raised concerns for weeks that Iranian-backed groups in Iraq were likely to attack U.S. forces. On Friday, more than 30 rockets were launched on an Iraqi base near the city of Kirkuk, killing the contractor and wounding several U.S. service members.

Lt. Col. Hassan Kadhim, an officer with the Iraqi army’s 8th Division, said he is concerned about the tensions between the United States and the Shiite-led Popular Mobilization Units, an umbrella organization of paramilitary groups that U.S. officials say receive support from Iran.

“If something happened, then we’ll be in the middle, and it will be chaos,” he said.

He said it is clear that recent attacks on U.S. bases in Iraq were not carried out by the Islamic State. “It’s being done by Iran proxies,” he said. Iran wants “to have their war in our land.”

Earlier this month, Defense Secretary Mark T. Esper asked Iraqi officials to respond to an uptick in attacks on U.S. bases in Iraq.

“My suspicion would be that Iran is behind these attacks, much like they’re behind a lot of malign behavior throughout the region, but it’s hard to pin down,” Esper told reporters. “So again, we need their help in terms of getting the security situation under control and stabilized, but we also still retain our right of self-defense.”

About 5,000 U.S. troops remain in Iraq at the invitation of the Iraqi government to train and advise Iraqi forces in their campaign against the Islamic State. President Trump has said he wants to end long-running wars in the Middle East but also has deployed thousands of additional troops in the region this year to counter Iran.

Salim reported from Baghdad.