Washington (United States) (AFP) – US forces struck more than 70 Islamic State group targets in Syria on Friday in what President Donald Trump described as "very serious retaliation" for an attack that killed three Americans last weekend.

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 -- that left two US soldiers and a US civilian dead.

In response, the United States "struck more than 70 targets at multiple locations across central Syria with fighter jets, attack helicopters, and artillery," US Central Command (CENTCOM) said in a statement.

"The operation employed more than 100 precision munitions targeting known ISIS infrastructure and weapons sites," CENTCOM said, using an acronym for the Islamic State group.

Trump said in a post on his Truth Social network that the United States is "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."

CENTCOM said that US and allied forces have "conducted 10 operations in Syria and Iraq resulting in the deaths or detention of 23 terrorist operatives" following the Palmyra attack, without specifying which groups the militants belonged to.

Syria's foreign ministry, while not directly commenting on the Friday strikes, said in a post on X that the country is committed to fighting the Islamic State (IS) group and "ensuring that it has no safe havens on Syrian territory, and will continue to intensify military operations against it wherever it poses a threat."

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 US 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 US personnel who were targeted were supporting Operation Inherent Resolve, the international effort to combat IS, 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 IS 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 US personnel in Syria in the following months, while US envoy for Syria Tom Barrack said in June that Washington would eventually reduce its bases in the country to one.

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