Over 100 people were killed in an apparent drone attack on a military academy in the Syrian province of Homs on Thursday.
Civilians were also among the dead, according to a statement from the Syrian Defense Ministry, which also claimed that terrorist groups were behind the attack.
The Syrian Observatory for Human Rights, a war-monitoring organization based in the UK, said that some 116 people had been killed, including 30 civilians.
The Syrian Health Ministry put the death toll at 81, adding that 239 were injured. The toll was not final, the ministry said.
Syria's defense and foreign ministries vowed in written statements to respond "with full force" to the attack.
United Nations chief Antonio Guterres was "deeply concerned" over the drone attack and the retaliatory shelling, his spokesman Stephane Dujarric said.
"Today's horrific scenes are a reminder of the need to immediately de-escalate violence, toward a nationwide ceasefire and a cooperative approach to countering Security Council-listed terrorist groups," said Geir Pedersen, the UN special envoy for Syria.
What we know about the attack
Syria's military said "armed terrorist organizations" had targeted "the graduation ceremony for officers of the military academy," according to state-run SANA news agency.
The attack included "explosive-laden drones," the military also said.
There were no immediate claims of responsibility for the attack, but the Syrian Observatory pointed to either the Islamist militia Tahrir al-Sham or the so-called "Islamic State" as possible suspects.
According to Reuters, the Syrian defense minister had been attending the graduation ceremony but left shortly before the attack.
"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," a man who had helped to set up decorations for the event told Reuters.
The army's general command condemned the attack as "cowardly" and vowed to "respond with full force."
Homs has been under the control of the government of Bashar Assad since 2017.