The man who killed three Americans in an attack in Syria on Saturday, which Washington blamed on the Islamic State organization, was a member of the Syrian security forces, from which he was to be "dismissed" for "extremist Islamist ideas", the Syrian Interior Ministry said on Sunday.

"The perpetrator of the attack was a member of the General Security forces under the Ministry of the Interior for more than ten months, and was stationed in several cities before being transferred to Palmyra," a Syrian security official told AFP, indicating that "11 members of the General Security forces" had been arrested after the attack.

Two American soldiers and a civilian were killed Saturday in the Palmyra desert region while on a mission, according to the Pentagon. The perpetrator of the attack was killed.

"We will retaliate," warned US President Donald Trump on Saturday, blaming the attack on the Islamic State (IS) group "in a very dangerous area of Syria, which is not totally controlled" by the Syrian government.

Syrian authorities suspected the killer of having "extremist Islamist ideas" and had decided to "dismiss" him from the security forces on Sunday, the day after the attack, Interior Ministry spokesman Noureddine al-Baba said in a statement to state television.

Three other American soldiers were wounded in the attack, according to the U.S. military command for the Middle East (Centcom).

Washington claimed that the Americans were killed in an "ambush by a lone gunman" from ISIS, which had previously controlled the Palmyra region before being defeated in Syria.

This is the first time such an attack has occurred in Syria since the takeover, a year ago, of an Islamist coalition that has forged closer ties with the United States.

The American contingent was in the area for a "mission to support ongoing operations against ISIS," said Pentagon spokesman Sean Parnell, specifying that the American civilian killed was an interpreter.

The Syrian government on Saturday condemned the "terrorist attack", which also wounded two members of the Syrian security forces, according to the official Sana news agency.

- Washington "warned" -

The spokesman for the Interior Ministry had stated on Saturday that the forces of the international anti-jihadist coalition, led by Washington, had not heeded warnings from Damascus regarding the risk of infiltration by the IS group.

"There were prior warnings from the internal security command to partner forces" in the Palmyra region, he told state television.

The security forces and army of ousted President Bashar al-Assad collapsed when an Islamist coalition seized power in Syria a year ago following a rebel offensive.

The new authorities have massively enlisted new recruits to form the General Security forces.

A new army has also been formed, which brings together members of factions allied with the authorities that had dissolved at their request.

The new interim president, Ahmad al-Sharaa, has broken with his jihadist past, and during his visit to Washington last month, Damascus joined the US-led international anti-jihadist coalition.

American forces in Syria are deployed in areas under Kurdish control in the north, as well as at the Al-Tanf base, near the Jordanian border.