Shafaq News/ The leader of al-Nujaba movement, Akram al-Kaabi, on Monday vowed to avenge the death of five of his fighters who were killed in a U.S. airstrike on Sunday.

In a statement, al-Kaabi said the strike was a "cowardly act" and that the United States would pay "a hefty price."

"The blood of our martyrs will not be shed in vain," al-Kaabi said. "We will continue to fight until we expel the American occupiers from Iraq."

The U.S. military said the airstrike was carried out in response to a rocket attack on a U.S. military base in Kirkuk. The Iraqi government condemned the airstrike, calling it a "violation of Iraqi sovereignty."