UN's Syria envoy deplores 'horrific' bloodshed after attack kills scores in Homs

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 regime-held central city of Homs killed more than 100 people, a war monitor said, with state media blaming "terrorist organisations".

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.

"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.

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 and militias. Most of those who have died have been civilians killed in regime bombing.