An attack Thursday on a Syrian military academy killed 112 people, a war monitor said, with state media blaming "terrorist organisations" for the drone strike in government-held Homs.
Separately, Turkish air raids in the country's Kurdish-held northeast killed at least 11 people, according to Kurdish forces, after Ankara had threatened retaliation for a bomb attack.
In the central city of Homs, "armed terrorist organisations" targeted "the graduation ceremony for officers of the military academy", an army statement carried by official news agency SANA said.
The Syrian Observatory for Human Rights, a Britain-based monitor, reported "112 dead including 21 civilians, 11 of them women and girls". It said at least 120 people were wounded.
Health Minister Hassan al-Ghobash told state television the "preliminary" toll was 80 dead "including six women and six children" and around 240 wounded.
There was no immediate claim of responsibility.
The attack was carried out with "explosive-laden drones", according to the military statement, vowing to "respond with full force". The government declared three days of mourning starting Friday.
In the rebel-held Idlib region, residents reported heavy bombardment by government forces in apparent retaliation. The Observatory said eight people had been killed and some 30 wounded.
Swathes of Idlib province are controlled by Hayat Tahrir al-Sham, led by Al-Qaeda's former Syria branch. The jihadist group has used drones to attack government-held areas in the past.
United Nations chief Antonio Guterres was "deeply concerned" over the drone attack and the retaliatory shelling, his spokesman Stephane Dujarric said.
Geir Pedersen, the UN special envoy for Syria, said in a statement: "Today's horrific scenes are a reminder of the need to immediately de-escalate violence, towards a nationwide ceasefire and a cooperative approach to countering Security Council-listed terrorist groups."
Overnight, Syrian shelling killed an elderly woman and four of her children in a rebel-held area of Aleppo province, rescue workers and the Observatory said.
- Turkish drone shot down -
Turkey's defence ministry said in a statement Thursday evening that Ankara forces had carried out air strikes in northern Syria, destroying 30 targets, including "shelters, depots and storage sites".
The Kurds' internal security forces said Turkey had carried out 21 strikes in the area, killing "11 people, including five civilians and six" security personnel.
Pentagon spokesman Pat Ryder told reporters that US F-16 warplanes over Syria had shot down a Turkish drone on Thursday, deeming it "a potential threat" after it approached "less than a half kilometre from US forces" near Hasakeh.
Turkey has carried out repeated strikes on targets in Syria's Kurdish-held northeast.
On Wednesday, Ankara warned it would step up its cross-border air raids, after concluding that militants who staged a weekend attack in the Turkish capital had come from Syria.
The US-backed Syrian Democratic Forces led the battle that dislodged Islamic State group fighters from their last scraps of territory in Syria in 2019.
Turkey views the Kurdish People's Protection Units (YPG) that dominate the SDF as an offshoot of the Kurdistan Workers' Party (PKK), listed as a terror group by Ankara and its Western allies.
Turkey has launched strikes on PKK positions in northern Iraq since Sunday's attack in Ankara, which wounded two Turkish security officers and was claimed by the Kurdish group.
AFP correspondents in northeastern Syria saw smoke rising from oil sites near Qahtaniyeh, close to the Turkish border.
Two power stations in the area were also hit, as well as the vicinity of a dam.
- 'Worsening' -
Farhad Shami, spokesman for the SDF, the Kurds' de facto army, said the strikes had targeted military and civilian sites.
"There has been a clear escalation since the Turkish threats," he said.
In the market of the city of Qamishli in Hasakeh province, vendors voiced concern.
"The situation is worsening every day. Turkey doesn't let us breathe," said Hassan al-Ahmad, a 35-year-old fabric merchant.
SDF commander Mazloum Abdi denied Wednesday that the Ankara assailants had "passed through our region".
"Turkey is looking for pretexts to legitimise its ongoing attacks on our region," he said.
The Kurdish administration urged "the international community" to "take a stand capable of dissuading" Turkey from its attacks.
The United States, Russia and Turkey all have troops in the country.
Between 2016 and 2019, Turkey carried out three major operations in northern Syria against Kurdish forces.
The conflict in Syria has killed more than half a million people since it began in 2011 with a brutal crackdown on anti-government protests, spiralling into a devastating war involving foreign armies, militias and jihadists.
US shoots down Turkish drone over Syria
US warplanes on Thursday shot down a Turkish drone that was deemed a threat to American forces in Syria, the Pentagon said -- an incident likely to raise tensions between the NATO allies.
The shootdown came as Turkey targeted Kurdish forces in Syria following a suicide bombing in Ankara that was claimed by the Kurdistan Workers' Party (PKK).
US troops observed drones carrying out strikes early Thursday, some of them inside a "restricted operating zone" (ROZ) near Hasakeh, about a kilometer (less than a mile) from American troops, Pentagon spokesman Brigadier General Pat Ryder told journalists.
A few hours later, a Turkish drone returned to the ROZ and flew toward American forces despite what a US official said were a dozen calls to Ankara's military.
Ryder said "US commanders assessed... the UAV, which was now less than a half kilometer from US forces, to be a potential threat," using an abbreviation for unmanned aerial vehicle.
"US F-16 fighters subsequently shot down the UAV in self-defense," he said.
Defense Secretary Lloyd Austin spoke with his Turkish counterpart Yasar Guler on Thursday, urging "de-escalation in northern Syria and the importance of maintaining strict adherence to de-confliction protocols and communication through established military-to-military channels," Ryder said in a statement.
Turkey's defense ministry confirmed the call, saying the two officials discussed "the latest developments in Syria."
The United States has approximately 900 troops deployed in Syria as part of international efforts to combat the Islamic State (IS) group, and carries out frequent raids targeting the jihadists.
- Dangerous mix -
US forces are partnered with the Syrian Democratic Forces (SDF), which led the fight that dislodged IS fighters from the last territory they held in the country in 2019.
But Turkey views the Kurdish People's Protection Units that dominate the SDF as an offshoot of the PKK, which is considered a terror group by Ankara and its Western allies.
While Turkey regularly carries out drone attacks in the semi-autonomous Kurdish region in northeast Syria, it warned of more intense cross-border air raids after concluding the militants who staged the Ankara bombing came from that country.
Turkish strikes on military and infrastructure targets in Kurdish-held areas of Syria killed at least nine people on Thursday, according to Kurdish security forces.
Syria contains a dangerous mix of often antagonistic forces operating on the ground and in the air, including those from President Bashar al-Assad's government, Russia, the United States, Turkey, Iran-linked groups, the Kurds and jihadist organizations.
Turkey has shot down both Syrian and Russian planes in the past, while the United States more recently accused Moscow's forces of repeatedly harassing its aircraft over Syria.
In July, Washington said a Russian fighter aircraft dropped flares above an American drone as it took part in an anti-IS mission, damaging its propeller.
That incident followed others in which the United States said Russian planes harassed a manned American reconnaissance plane as well as MQ-9 drones.