Syrian who killed Americans was part of security forces - ministry

Syria's interior ministry said Sunday that the gunman who killed three Americans in the central Palmyra region the previous day was a member of the security forces who was set to be fired for extremism.

Two US troops and a civilian interpreter died in what the Syrian government described as a "terrorist attack" on Saturday, while Washington said it had been carried out by a Daesh (ISIS) militant who was subsequently killed.

The Syrian authorities "had decided to fire him" from the security forces before the attack for holding "extremist ideas" and had planned to do so on Sunday, interior ministry spokesman Noureddine al-Baba told state television.

A Syrian security official told AFP on Sunday that "11 members of the general security forces were arrested and brought in for questioning after the attack".

The official, who spoke on condition of anonymity, said the gunman had belonged to the security forces "for more than 10 months and was posted to several cities before being transferred to Palmyra".

US President Donald Trump vowed "very serious retaliation" following Saturday's attack.

A Syrian defence ministry official told AFP on condition of anonymity that prior to the attack, US forces had "arrived by land from the direction of the Al-Tanf military base" in southeastern Syria, near the border with Jordan.

"The joint Syrian-American delegation first toured the city of Palmyra, then proceeded to the T-4 airbase before returning to a base in Palmyra," the source added.

A Syrian military official who requested anonymity said on Saturday that the shots were fired "during a meeting between Syrian and American officers" at a Syrian base in Palmyra.

However, a Pentagon official speaking on condition of anonymity told AFP that the attack "took place in an area where the Syrian president does not have control".