U.S. military forces used “precision defense strikes” against Iranian-backed militia in Iraq and Syria on Sunday in response to a string of attacks on Iraqi bases where some American service members were being housed, according to the Pentagon.

“In response to repeated Kata’ib Hizbollah (KH) attacks on Iraqi bases that host Operation Inherent Resolve coalition forces, U.S. forces have conducted precision defensive strikes against five KH facilities in Iraq and Syria that will degrade KH’s ability to conduct future attacks against OIR coalition forces,” chief Pentagon spokesman Jonathan Hoffman said in a statement.

“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,” he further added.

The three locations in Iraq and two in Syria were controlled by Iranian-backed Shiite Muslim militia, according to the Pentagon. The strategic locations included command and control centers and weapon storage facilities, but it was not made clear what U.S. assets were used in the strikes.

Tensions have remained high in the region, and the strikes were in response to military action by Kata’ib Hezbollah forces against military bases in Iraq that were also housing American troops. A U.S. civilian contractor was killed during a rocket attack on an Iraqi base close to Kirkuk on Friday.

U.S. Secretary of State Mike Pompeo has been a vocal opponent of the action in Iraq, and earlier this month blamed Iranian-backed forces for attacks in the region. He added that any future attacks on Americans or their allies would be “answered with a decisive U.S. response.”

“As long as its malign behaviors continue, so will our campaign of maximum pressure,” Pompeo said during a press conference on Dec. 11.

The weekend’s action was only the latest as relations between Washington and Tehran continue to crumble. In June, the U.S. hit Iran with new sanctions after placing blame on the nation for shooting down a U.S. surveillance drone.

Tensions remained high in September after Washington blamed Iran for strikes against Saudi Arabian oil fields that forced a temporary shut down of over half the country’s oil production. Saudi Arabia’s defense ministry placed blame on Iran after finding debris after the attacks.