Syrian regime vowed to respond "with full force" following the drone attack/ Photo: AP Photo: AP

A drone attack has hit a crowded military graduation ceremony in the Syrian city of Homs, killing 80 people and wounding 240, in one of the deadliest recent attacks on the regime army, which has been fighting a civil war for more than a decade.

The strike on Thursday killed civilians, including six children, as well as military personnel, and there were concerns the death toll could rise as many of the wounded were in serious condition, regime Health Minister Hassan al Ghabash said.

Syria's military said in an earlier statement that drones laden with explosives targeted the ceremony packed with young officers and their families as it was wrapping up.

Without naming any particular group, the military accused insurgents "backed by known international forces" of the attack and said, "it will respond with full force and decisiveness to these terrorist organisations, wherever they exist."

No group immediately claimed responsibility for the attack as Syria endures its 13th year of conflict.

UN Secretary General Antonio Guterres "expressed deep concern" about the drone attack in Homs as well as reports of retaliatory shelling in northwest Syria, his spokesperson Stephane Dujarric said.

Damascus announced a three-day state of mourning starting on Friday.

Idlib targeted

The city of Homs is deep in regime-held territory, far from the front lines where regime and opposition forces routinely skirmish.

After the drone attack, the Syrian forces shelled villages in Idlib province, in the opposition-held northwest.

In the towns of Al Nayrab and Sarmin east of Idlib city, at least 10 civilians were wounded, according to opposition-held northwestern Syria's civil defence organisation known as the White Helmets.

Regime forces continued to shell other areas in the opposition-held enclave.

The Syrian military shelled another village in the region earlier on Thursday before the drone attack over Homs, killing at least five civilians, activists and emergency workers said.

The shelling hit a family house on the outskirts of the village of Kafr Nouran in western Aleppo province, according to the White Helmets.

Years of war and displacement

The vast majority of around 4.1 million people residing in northwestern Syria live in poverty, relying on humanitarian aid to survive.

The civil war started with peaceful protests against Syria's Bashar al Assad's rule in March 2011, but quickly morphed into a full-blown civil war after Assad's brutal crackdown on the protesters.

The tide turned in Assad's favour against opposition groups in 2015, when Russia provided key military backing to Syria.

So far, the war has killed half a million people, wounded hundreds of thousands and left many parts of the country destroyed.

It has displaced half of Syria's prewar population of 23 million, including more than 5 million who are refugees outside Syria.

AP