Two US troops and a civilian interpreter died in the attack on Saturday, which the US Central Command said had been carried out by an alleged Islamic State group (IS) 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”.

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 once controlled by IS during the height of its territorial expansion in Syria.

The incident is the first of its kind reported since Islamist-led forces overthrew longtime Syrian ruler Bashar al-Assad in December last year, and rekindled the country’s ties with the United States.

Pentagon spokesman Sean Parnell said the soldiers “were conducting a key leader engagement” in support of counter-terrorism operations when the attack occurred, while US envoy to Syria Tom Barrack said the ambush targeted “a joint US–Syrian government patrol”.

US President Donald Trump called the incident “an ISIS attack against the US, and Syria, in a very dangerous part of Syria, that is not fully controlled by them”, using another term for the group.

He said the three other US troops injured in the attack were “doing well”.