An attack on a Syrian military academy killed more than 110 people, a war monitor said, with state media blaming "terrorist organisations" for the deadly drone strike in government-held Homs.
Separately, Turkish air raids in the war-torn country's Kurdish-held northeast killed at least 11 people, according to Kurdish forces, after Ankara had threatened raids in retaliation for a bomb attack.
In the central Syrian 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 tomorrow.
In the rebel-held Idlib region, residents reported heavy bombardment by government forces in apparent retaliation. The Observatory said eight people were killed and some 30 wounded.
Swathes of Idlib province are controlled by Hayat Tahrir al-Sham, led by the former local Al-Qaeda branch.
The jihadist group has used drones to attack government-held areas in the past.
United Nations Secretary-General Antonio Guterres "is deeply concerned about the drone attack", his spokesman Stephane Dujarric said.
The UN chief "is also deeply concerned about reports of retaliatory shelling" in northwest Syria, Mr Dujarric added.
Overnight, Syrian army 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 'escalation'
Meanwhile, the Turkish strikes on Hasakeh province in Kurdish-held northeast Syria "killed six members of the internal security" agency, a statement from the Kurdish force's media centre said.
A worker at a site in the province was also killed, according to Farhad Shami, spokesman for the Kurdish-led Syrian Democratic Forces (SDF), the Kurds' de facto army.
The Kurdish authorities' statement also said "two civilians" were killed in a strike on a motorcycle.
Turkey regularly strikes targets in Syrian Kurds' semi-autonomous region.
On Wednesday, Ankara warned of more intense cross-border air raids, after concluding that militants who staged a weekend attack in the Turkish capital came from Syria.
The US-backed SDF led the battle that dislodged Islamic State (IS) group fighters from their last scraps of Syrian territory in 2019.
Turkey views the Kurdish People's Protection Units (YPG) that dominate the SDF as an offshoot of the Kurdistan Workers' Party (PKK), which is listed as a terror group by Turkey and its Western allies.
Since Sunday's Ankara attack, which wounded two Turkish security officers and was claimed by the PKK, Ankara has launched strikes on the Kurdish group's positions in northern Iraq.
AFP correspondents in Syria's northeast saw black smoke rising from oil sites near Qahtaniyeh, close to the Turkish border.
Two power stations in the area were also hit, the correspondents said.
The SDF's Mr Shami said the strikes had targeted military and civilian sites.
"There has been a clear escalation since the Turkish threats," he said, reporting intensive overflights of Kurdish-held areas in northeast Syria.
'Legitimate targets'
Turkish Foreign Minister Hakan Fidan had warned of reprisals against Kurdish fighters in northeastern Syria in the aftermath of Sunday's attack outside the interior ministry in Ankara.
He had alleged the perpetrators "came from Syria and were trained there".
"From now on, all infrastructure, large facilities and energy facilities belonging to (armed Kurdish groups) in Iraq and Syria are legitimate targets for our security forces," Mr Fidan had said in televised comments.
In the market of the city of Qamishli in Hasakeh province, vendors were anxiously following the escalation on their mobile phones and televisions.
"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 yesterday denied that the Ankara assailants had "passed through our region".
"Turkey is looking for pretexts to legitimise its ongoing attacks on our region and to launch a new military aggression," he said.
The Kurdish administration today called on "the international community, the international coalition" and Russia to "take a stand capable of dissuading" Turkey from its attacks.
State Department spokesperson Vedant Patel said the United States "remains concerned about the military escalation in northern Syria".
The United States, Russia and Turkey all have troops in areas of the war-torn 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 complex battlefield involving foreign armies, militias and jihadists.
US shoot down Turkish drone over Syria
US F-16 warplanes shot down a drone belonging to NATO ally Turkey that was deemed a potential threat to American forces in Syria, the Pentagon said.
US forces observed drones carrying out air strikes in northeast Syria, some of them inside a "restricted operating zone" (ROZ) near Hasakah, about 1km from American troops, Pentagon spokesman Brigadier General Pat Ryder told journalists.
A few hours later, a Turkish drone returned to the ROZ heading toward US forces.
"US commanders assessed that the UAV, which was now less than a half kilometer from US forces, to be a potential threat and US F-16 fighters subsequently shot down the UAV in self-defense," he said, using an abbreviation for unmanned aerial vehicle.
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.
