A drone attack on a Syrian military academy in the western city of Homs has killed at least 100 people, a monitoring group says.
The explosive-laden drones targeted a graduation ceremony attended by cadets' families, and women and children were among the dead.
The army blamed "terrorist groups backed by known international forces".
There was no immediate claim from the rebels and jihadists battling the government in the country's civil war.
The drone attack is believed to have been launched from opposition-held areas north-west of Homs.
Later, first responders from the White Helmets reported that five civilians had been killed in intense government artillery and missile strikes on several cities, towns and villages in the opposition stronghold of Idlib province.
Syria's state news agency, Sana, quoted a statement from the General Command of the Armed Forces as saying that several drones carrying explosives targeted the Homs military academy just after the afternoon graduation ceremony had ended.
The statement said the armed forces "considers this act an unprecedented criminal one, and affirms that it will respond with full force and determination to these terrorist groups wherever they are".
In a later report the agency quoted Health Minister Hassan al-Gabbash saying the strike had injured more than 200 people and that six women and six children were among the dead.
A man who had earlier helped set up decorations at the site said: "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 graphic video of the aftermath of the attack showed dozens of casualties and their relatives screaming for help inside a large, walled parade ground. Gunfire can also be heard in the background.
The Syrian Observatory for Human Rights (SOHR) reported that Syria's defence minister attended the graduation ceremony but left minutes before the attack.
More than half a million people have been killed by the civil war that erupted after President Bashar al-Assad cracked down violently on peaceful pro-democracy demonstrations in 2011.
Some 6.8 million people are internally displaced, while another six million are refugees or asylum-seekers abroad.
The UN's special envoy for Syria, Geir Pedersen, described the attack on the academy as "horrific" and called on all parties to the conflict to "exercise the utmost restraint".
"All sides must respect their obligations under international law and ensure the protection of civilians and civilian infrastructure," he said
"Today's developments further highlight that the status quo in Syria is unsustainable and that, in the absence of a meaningful political path... I fear we will only see further deterioration, including in the security situation."
In a separate development in Syria on Thursday, at least 10 people were reportedly killed in Turkish drone strikes in a Kurdish-controlled region of north-eastern Syria that were prompted by a bomb attack in Ankara claimed by Kurdish militants.
The SOHR said 17 sites were targeted, including facilities affiliated with the Syrian Democratic Forces (SDF), a US-backed, Kurdish-led militia alliance, as well as a power station in Qamishli, a water station near Hassakeh and an oil field.
The US military also shot down an armed Turkish drone that was operating near its troops in Syria, a US official told Reuters.
