Baghdad (dpa) – An Iraqi Shiite militia leader has threatened a "harsh response" against the US forces in Iraq after airstrikes on his Iranian-backed group that killed 25 people on Sunday night.

"The blood of martyrs and those wounded will not be in vain and our response will be harsh against the US forces in Iraq," the deputy chief of the powerful militia Hashd Shaabi (Popular Mobilization Forces), Abu Mahdi al-Mohandes, said in a statement carried by the website of al-Sumaria TV late on Sunday.

On Sunday, the US military said it carried out "precision defensive strikes" in Iraq and Syria against the Kataib Hezbollah militia group, which was blamed for an attack earlier this week that killed a US citizen.

The strikes targeted five sites in total, three in Iraq and two in Syria, the Pentagon said in a statement.

Hashd Shaabi said on Monday that the US airstrike left 25 militiamen dead and 50 others wounded.

On Monday, Lebanon's Shiite Hezbollah group condemned the US airstrikes, calling them a blatant attack on Iraq's sovereignty, security and stability and on the Iraqi people.

Lebanon's Hezbollah and Iraq's Kataib Hezbollah, a Hashed Shaabi faction, are both backed by Iran.

Iraq’s influential Shiite cleric Ali al-Sistani denounced the attacks on Monday and called for respect for Iraq's sovereignty.

Iraqi President Barham Salih said the attacks were a violation of Iraq's sovereignty, calling for his country not to be dragged into regional and international conflicts.

The US strikes came after a rocket attack on an Iraqi base near Kirkuk that resulted in the death of a civilian contractor, four injured US service members and two injured members of the Iraqi Security Forces.