The UN's Syria envoy said Thursday's "horrific scenes" in Syria showed the status quo in the war-torn country was unsustainable — and warned the security situation could worsen further still.

An attack on a Syrian military academy in the government-held central city of Homs killed more than 100 people, a war monitor said, with state media blaming "terrorist organizations".

Separately, Turkish air raids in the Kurdish-held northeast killed at least nine people, according to Kurdish forces, after Ankara had threatened raids in retaliation for a bomb attack.

"I am gravely concerned by the escalating violence in Syria today," Geir Pedersen, the UN Special Envoy for Syria, said in a statement issued in Geneva.

"I deeply deplore the loss of life on all sides. I urgently appeal to all sides to exercise the utmost restraint.

"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.

"All sides must respect their obligations under international law and ensure the protection of civilians and civilian infrastructure."

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, spiraling into a complex battlefield involving foreign armies, militias, and jihadists.

"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," said Pedersen.