US airstrikes on Iran-backed militia in Iraq and Syria has killed at least 25 fighters. Kataeb Hezbollah vowed to exact revenge after US Defense Secretary Mark Esper said Washington had carried out military strikes targeting the Iran-backed Iraqi militia blamed for a rocket attack that killed an American contractor in Iraq last week. Over 50 are said to have sustained injuries.

The US military said “precision defensive strikes” were conducted against five sites of Kataeb Hezbollah, or Hezbollah Brigades in Iraq and Syria. Kataeb Hezbollah said in a statement, “Our battle with America and its mercenaries is now open to all possibilities. We have no alternative today other than confrontation and there is nothing that will prevent us from responding to this crime.”

The US blames the militia for a rocket barrage on Friday that killed a US defense contractor at a military compound near Kirkuk, in northern Iraq. Officials said as many as 30 rockets were fired in that attack.

Iraq’s Hezbollah Brigades, a separate force from the Lebanese militant group Hezbollah, operates under the umbrella of the state-sanctioned militias known collectively as the Popular Mobilization Forces.