The US confirmed it carried out military strikes in Iraq and Syria targeting an Iranian-backed Iraqi militia blamed for a rocket attack that killed an American contractor, Defense Secretary Mark Esper said.

US Secretary of State Mike Pompeo said the strikes send the message that the US will not tolerate actions by Iran that jeopardize American lives.

“Precision defensive strikes” were conducted against five sites of Kataeb Hezbollah, or Hezbollah Brigades, Defense Department spokesman Jonathan Hoffman said in a statement.

The US blames the militia for a rocket barrage 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 Friday’s assault.

Esper said the US hit three of the militia’s sites in western Iraq and two in eastern Syria, including weapon depots and the militia’s command and control bases.

The strike is the first direct confrontation between US and Iranian-backed forces in Iraq since 2011, when President Obama withdrew some of his forces.

"I would note also that we will take additional actions as necessary to ensure that we act in our own self-defence and we deter further bad behavior from militia groups or from Iran," said Defence Secretary Mark Esper.

The Popular Mobilization Forces said Sunday that the U.S. strikes killed at least 25 of Kataeb Hezbollah's members.