Damascus, Syria - As President Donald Trump vowed vengeance for the US troops killed in Syria Saturday, Damascus revealed the gunman was member of its 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 an ISIS militant who was then killed.

The Syrian authorities "had decided to fire him" from the security forces before the attack for holding "extremist Islamist 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."

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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".

Palmyra, home to UNESCO-listed ancient ruins, was controlled by ISIS at the height of its territorial expansion in Syria.

Trump vowed to hit back following Saturday's attack.

"The President of Syria, Ahmed al-Sharaa, is extremely angry and disturbed by this attack. There will be very serious retaliation," he wrote on social media, adding that the three other US troops injured in the incident were "doing well".

The official SANA news agency said the attack also wounded two members of the Syrian security forces.

As the leader of an Islamist group formerly tied to Al-Qaeda, al-Sharaa toppled dictator Bashar al-Assad a year ago. Under his regime, Syria has officially joined the international coalition against ISIS.