An attack on a military academy in Syria on Thursday resulted in up to 100 deaths, according to a war monitor and an official, as weaponised drones bombed the location shortly after Syria’s defence minister left a graduation ceremony there.

In a nation that has endured a 12-year civil war, the deployment of weaponised drones was unprecedented, making it one of the most lethal attacks ever against a Syrian army installation.

The victims of the attack on the military academy in the central province of Homs included civilians and military personnel, Syria’s defence ministry said, adding that “terrorist” groups had used drones.

However, the statement did not specify an organisation and no group immediately claimed responsibility for the attack.

Following Thursday’s drone attack, Syrian government forces carried out heavy bombing attacks on the opposition-held zone of Idlib throughout the day, as Syria’s defence and foreign ministries vowed to respond “with full force”.

Syria’s defence minister attended the graduation ceremony but left minutes before the attack, according to a Syrian security source and a security source in the regional alliance backing the Damascus government against opposition groups, Reuters reported.

“After the ceremony, people went down to the courtyard and the explosives hit. We don’t know where it came from, and corpses littered the ground,” said a Syrian man who had helped set up decorations at the academy for the occasion.

The Syrian Observatory for Human Rights said more than 100 people were killed and 125 injured. An official in the alliance backing Syria’s government said the toll was around 100.

Health Minister Hassan Al-Ghobash gave a lower death count, telling state TV 80 people had been killed, including six women and six children, but said about 240 people had been injured.

Syria’s conflict began with protests against President Bashar al-Assad in 2011 but spiralled into an all-out war that has left hundreds of thousands dead and millions displaced.

The Syrian army has been gutted by the fighting and relied heavily on military support from Russia and Iran as well as Tehran-backed fighters from Lebanon, Iraq and other countries.

Assad regained most of the country, but a swathe in the north bordering Turkey is still held by armed opposition groups, including hardline jihadist fighters.

Additional input from Reuters