At least 60 people have been killed in an attack on a military academy in Syria, a war monitor and a security source said, with weaponised drones bombing the site minutes after Syria’s Defence Minister left a graduation ceremony there.

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Civilians and military personnel were killed in the attack on the military academy in the central province of Homs, Syria’s Defence Ministry said in a statement, adding “terrorist” groups had used drones to carry it out.

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

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.

The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said at least 78 people had been killed and more than 140 wounded.

The source in the alliance backing Syria’s government said the toll was 66.

The military accused insurgents “backed by known international forces” of the attack, without naming any particular group, and said that some of the wounded were in critical condition, including women and children.

The Syrian military said “it will respond with full force and decisiveness to these terrorist organisations, wherever they exist”.

Syria’s state television said the government has announced a three-day state of mourning, starting on Friday.

“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.

Footage shared with Reuters 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 smouldering 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.

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.

Reuters