WASHINGTON (AP) — Defense Secretary Pete Hegseth has announced the start of an operation to “eliminate ISIS fighters, infrastructure, and weapons sites” in Syria following the deaths of three U.S. citizens.

“This is not the beginning of a war — it is a declaration of vengeance. The United States of America, under President Trump’s leadership, will never hesitate and never relent to defend our people,” he said Friday on social media.

Two Iowa National Guard members and a U.S. civilian interpreter were killed Dec. 13 in an attack in the Syrian desert that the Trump administration has blamed on the Islamic State group. The slain National Guard members were among hundreds of U.S. troops deployed in eastern Syria as part of a coalition fighting IS.

Soon after word of the deaths, President Donald Trump pledged “very serious retaliation” but stressed that Syria was fighting alongside U.S. troops. Trump has said Syrian President Ahmad al-Sharaa was “extremely angry and disturbed by this attack” and the shooting attack by a gunman came as the U.S. military is expanding its cooperation with Syrian security forces.

Syrian state television reported that strikes hit targets in rural areas of Deir ez-Zor and Raqqa provinces and in the Jabal al-Amour area near Palmyra. It said they targeted “weapons storage sites and headquarters used by ISIS as launching points for its operations in the region.”

A U.S. official told The Associated Press that the attack was conducted using F-15 Eagle jets, A-10 Thunderbolt ground attack aircraft and AH-64 Apache helicopters. The official, who spoke on condition of anonymity to discuss sensitive operations, said more strikes should be expected.

When asked for further information, the Pentagon referred AP to Hegseth’s social media post.

White House officials noted that Trump had made clear that retaliation was coming.

“President Trump told the world that the United States would retaliate for the killing of our heroes by ISIS in Syria, and he is delivering on that promise,” White House deputy press secretary Anna Kelly said in a statement.

Trump this week met privately with the families of the slain Americans at Dover Air Force Base in Delaware before he joined top military officials and other dignitaries on the tarmac for the dignified transfer, a solemn and largely silent ritual honoring U.S. service members killed in action.

The guardsmen killed in Syria on Saturday were Sgt. Edgar Brian Torres-Tovar, 25, of Des Moines, and Sgt. William Nathaniel Howard, 29, of Marshalltown, according to the U.S. Army. Ayad Mansoor Sakat, of Macomb, Michigan, a U.S. civilian working as an interpreter, was also killed.

The shooting nearly a week ago near the historic city of Palmyra also wounded three other U.S. troops as well as members of Syria’s security forces, and the gunman was killed. The assailant had joined Syria’s internal security forces as a base security guard two months ago and recently was reassigned because of suspicions that he might be affiliated with IS, Interior Ministry spokesperson Nour al-Din al-Baba has said.

The man stormed a meeting between U.S. and Syrian security officials who were having lunch together and opened fire after clashing with Syrian guards.

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Associated Press writer Abby Sewell in Beirut, Lebanon, contributed.