A drone attack has hit a crowded military graduation ceremony in the Syrian city of Homs, killing 80 people and wounding 240, the health minister said, in one of the deadliest attacks on the war-torn country’s army in years.

The strike killed civilians including six children, as well as military personnel, and there were concerns the death toll could rise as many of the wounded are in serious condition, health minister Hassan al-Ghabash said.

Syria’s military said drones laden with explosives targeted the ceremony packed with young officers and their families as it was wrapping up. It accused insurgents “backed by known international forces” of the attack, without naming any particular group, and said it will “respond with full force and decisiveness to these terrorist organisations, wherever they exist”.

No group immediately claimed the latest attack in the country’s 13-year conflict.

The UN chief “expressed deep concern” about the attack as well as reports of retaliatory shelling in north-west Syria, his spokesman Stephane Dujarric said.

“UN secretary-general Antonio Guterres condemned all violence and called for a nationwide ceasefire,” Mr Dujarric added.

The military did not provide any casualty numbers but state television said the government had announced a three-day state of mourning starting Friday.

Syria’s crisis started with peaceful protests against President Bashar Assad’s government in March 2011 but quickly morphed into a full-blown civil war after the government’s brutal crackdown on protesters.

The tide turned in his favour in 2015 when Russia provided key military backing to Syria, as well as Iran and the Lebanese militant group Hezbollah.

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 pre-war population of 23 million, including more than five million who are refugees outside Syria.

While most Arab governments have restored ties with the government in Damascus, Syria remains divided, with a north-west enclave under the control of al Qaida-linked militants from the Hayat Tahrir al-Sham group and Turkish-backed opposition fighters. The country’s north east is under control of US-backed Kurdish-led Syrian Democratic Forces.

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

After the drone attack, Syrian government forces shelled villages in Idlib province, in the rebel-held north west. In the towns of Al-Nayrab and Sarmin, east of Idlib city, at least 10 civilians were wounded, according to opposition-held north-western Syria’s civil defence organisation known as the White Helmets.

The Syrian army 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 the village of Kafr Nouran in western Aleppo province, according to the White Helmets.

A woman and four of her children were killed, according to the Syrian Observatory for Human Rights. Nine other members of the family were wounded, it said.

In north-eastern Syria, local authorities said Turkish drone attacks struck Hassakeh and Qamishli provinces on Thursday, hitting oil production facilities, electrical substations and a dam. A statement from local Kurdish authorities said six members of their security forces and five civilians were killed.

Meanwhile, three US officials told the Associated Press that a US F-16 fighter jet had shot down a Turkish drone on Thursday that came too close to their positions in Hassakeh after it had been dropping bombs in nearby areas.