A drone attack hit a crowded military graduation ceremony Thursday in the Syrian city of Homs, killing 80 people and wounding 240, the health minister said, in one of the deadliest recent attacks on an army that’s been fighting a civil war for more than a decade.

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 were in serious condition, 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 organizations, wherever they exist.”

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

U.N. 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. Guterres condemned all violence and called for a nationwide cease-fire, the spokesperson added.

The military did not provide any casualty numbers, but Syria's state television said the government announced a three-day state of mourning starting Friday. The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, and the pro-government Sham FM radio station reported the strikes earlier.

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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 the protesters. The tide turned in Assad’s favor against rebel groups in 2015, when Russia provided key military backing to Syria, as well as Iran and the Lebanese militant group Hezbollah.

So far, the war has killed half a million people, wounded hundreds of thousands and destroyed many parts of the country. It has displaced half of Syria’s prewar population of 23 million, including more than 5 million who are refugees outside Syria.

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

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

After the drone attack, Syrian government forces shelled villages in Idlib province, in the northwest. At least 10 civilians were wounded in the towns of Al-Nayrab and Sarmin east of Idlib city, according to opposition-held northwestern Syria’s civil defense organization known as the White Helmets. Government forces continue to shell other areas in the enclave.

The Syrian army shelled another village in the region earlier 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.

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

The vast majority of around 4.1 million people residing in northwestern Syria live in poverty, relying on humanitarian aid to survive. Many of them are Syrians, internally displaced by the war from other parts of the country.

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

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Meanwhile, three U.S. officials told The Associated Press that a U.S. F-16 fighter jet shot down a Turkish drone Thursday that came too close to their positions in Hassakeh after it had been dropping bombs in nearby areas. The U.S. has about 900 troops in Syria conducting missions to counter Islamic State group militants.

Turkey didn't immediately comment on the strikes but Ankara said the main Syrian Kurdish militia is allied with Turkey’s outlawed Kurdistan Workers’ Party, which has led an insurgency against Turkey since 1984 that has killed tens of thousands of people. Ankara has declared the PKK a terrorist group.

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Syrian Kurdish forces were a major U.S. ally in the war against the militant Islamic State group, which was defeated in Syria in March 2019.