U.S. forces on Friday launched a major military operation against the Islamic State group in Syria that President Donald Trump described as "very serious retaliation" for an attack that killed three Americans — two soldiers and a civilian.

Washington said a lone gunman from the militant group carried out the December 13 attack in Palmyra, home to UNESCO-listed ancient ruins and once controlled by jihadist fighters, which also wounded three U.S. service members.

"U.S. forces commenced OPERATION HAWKEYE STRIKE in Syria to eliminate ISIS fighters, infrastructure, and weapons sites in direct response to the attack on U.S. forces that occurred on December 13th in Palmyra, Syria," U.S. Defense Secretary Pete Hegseth posted on X, using an acronym for the jihadist group.

"This is not the beginning of a war — it is a declaration of vengeance," he wrote, adding: "Today, we hunted and we killed our enemies. Lots of them. And we will continue."

Trump said in a post on his Truth Social network that U.S. forces are "inflicting very serious retaliation, just as I promised, on the murderous terrorists responsible," and that those who attack Americans "WILL BE HIT HARDER THAN YOU HAVE EVER BEEN HIT BEFORE."

U.S. Central Command announced that American forces had "commenced a large-scale strike against ISIS infrastructure and weapons sites in Syria," while a security source in the Middle Eastern country said air strikes had targeted ISIS group positions in the Homs desert and rural areas of Deir Ezzor and Raqqa.

Syria's foreign ministry, while not directly commenting on the U.S. strikes, said in a post on X that the country is committed to fighting ISIS and "ensuring that it has no safe havens on Syrian territory."

The Americans killed in the Palmyra attack last weekend were Iowa National Guard sergeants William Howard and Edgar Torres Tovar, and Ayad Mansoor Sakat, a civilian from Michigan who worked as an interpreter.

Trump, Hegseth and top military officer General Dan Caine were among the U.S. officials who attended a somber ceremony marking the return of the dead to the United States on Wednesday.

The attack was the first such incident since the overthrow of longtime ruler Bashar al-Assad in December last year, and Syrian interior ministry spokesman Noureddine al-Baba said the perpetrator was a security forces member who was due to be fired for his "extremist Islamist ideas."

The U.S. personnel who were targeted were supporting Operation Inherent Resolve, the international effort to combat ISIS, which seized swaths of Syrian and Iraqi territory in 2014.

The jihadists were ultimately defeated by local ground forces backed by international air strikes and other support, but ISIS still has a presence in Syria, especially in the country's vast desert.

Trump has long been skeptical of Washington's presence in Syria, ordering the withdrawal of troops during his first term but ultimately leaving American forces in the country.

The Pentagon announced in April that the United States would halve the number of U.S. personnel in Syria in the following months, while U.S. envoy for Syria Tom Barrack said in June that Washington would eventually reduce its bases in the country to one.

U.S. forces are currently deployed in Syria's Kurdish-controlled northeast as well as at Al-Tanf near the border with Jordan.

(With input from AFP)