At least 100 people were killed on Thursday in an attack on a military academy in Syria, a war monitor and an official said, with weaponized drones bombing the site minutes after Syria’s defense minister left a graduation ceremony there.

It was one of the bloodiest attacks ever against a Syrian army installation, and unprecedented in its use of weaponized drones in a country which has faced 12 years of civil war.

Civilians and military personnel were killed in the attack on the military academy in the central province of Homs, Syria’s defense ministry said, adding “terrorist” groups had used drones.

The statement did not specify an organization and no group immediately claimed responsibility for the attack.

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

A videograb of the killing field.

Syria’s defense 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.

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

Footage shared through the messaging app WhatsApp showed people — some in fatigues and others in civilian clothes — lying in pools of blood in a large courtyard.

Some of the bodies were smoldering and others were still on fire. Amid the screaming, someone could be heard shouting “put him out!” A spray of gunfire could be heard in the background. Reuters could not authenticate the footage.

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 spiraled 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.