An Iranian-backed militia said Monday that the death toll from U.S. military strikes in Iraq and Syria against its fighters has risen to 25, and it's vowing to exact revenge for the "aggression of evil American ravens." The announcement in Baghdad came a day after U.S. Defense Secretary Mark Esper said Washington had carried out military strikes targeting the Iranian-backed Iraqi militia blamed for a rocket attack that killed an American contractor in Iraq last week.

Secretary of State Mike Pompeo said the strikes send the message that the U.S. won't tolerate actions by Iran that jeopardize American lives.

A hole left after an air strike is seen at headquarters of Kataib Hezbollah militia group in Qaim, Iraq, on December 30, 2019. STRINGER / REUTERS

The U.S. military said "precision defensive strikes" were conducted against five sites of Kataeb Hezbollah, or Hezbollah Brigades in Iraq and Syria.

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"Our battle with America and its mercenaries is now open to all possibilities," Kataeb Hezbollah said in a statement around midnight Sunday. "We have no alternative today other than confrontation and there is nothing that will prevent us from responding to this crime."

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

Destroyed headquarters of Kataib Hezbollah militia group are seen after in an air strike in Qaim, Iraq, on December 30, 2019. STRINGER / REUTERS

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. Many of them are supported by Iran.

The Popular Mobilization Forces said Sunday that the U.S. strikes killed at least 19 of Kataeb Hezbollah's members. But Kataeb Hezbollah spokesman Mohammed Mohieh told The Associated Press on Monday that the death toll rose to 25. At least 51 militiamen were wounded and some of them were serious condition, he said, adding that the militia group's commanders would decide on the retaliation.

In Tehran, foreign ministry spokesman Abbas Mousavi condemned the U.S. strikes against Kataeb Hezbollah as an "obvious case of terrorism" and accused Washington of ignoring Iraq's sovereignty.

The U.S. "must accept consequences for its illegal act," he said in a statement quoted by Agence France Presse. Mousavi said the presence of foreign troops is causing insecurity and tensions in the region.

"America must put an end to its occupying presence," said Mousavi.

Lebanon's Iran-backed Hezbollah also blasted the "brutal American aggression," saying those who took the decision to carry out the attack "will soon discover how stupid this criminal decision was."

Kataeb Hezbollah is led by Abu Mahdi al-Muhandis, one of Iraq's most powerful men. He once battled U.S. troops and is now the deputy head of the Popular Mobilization Forces. In 2009, the State Department linked him to the elite Quds Force of Iran's Revolutionary Guard, designated a foreign terrorist organization by President Trump earlier this year.

"The blood of the martyrs will not be in vain and our response will be very tough on the American forces in Iraq," al-Mohandes, said late Sunday, according to the Reuters news agency.

The U.S. maintains some 5,000 troops in Iraq, at the invitation of the Iraqi government to assist and train in the fight against the Islamic State of Iraq and Syria (ISIS).

The attack that killed the American contractor and U.S. counter-strikes come as months of political turmoil roil Iraq. About 500 people have died in anti-government protests, most of them demonstrators killed by Iraqi security forces.

The mass uprisings prompted the resignation last month of Prime Minister Adel Abdul-Mahdi, who remains in a caretaker capacity.

In a statement, Abdul-Mahdi said Esper had called him about a half-hour before the U.S. strikes on Sunday to tell him of U.S. intentions to hit bases of the militia suspected of being behind Friday's rocket attack. Abdul-Mahdi said he asked Esper to call off the U.S. plan.