More than 80 people were killed, according to media reports. The victims were both military personnel and civilians, including children. Hundreds more were wounded.

UN chief concerned

UN Secretary-General António Guterres was deeply concerned about the attack and “retaliatory shelling” across multiple locations in Syria, and underlined the need for a nationwide ceasefire.

“He also recalls that civilians and civilian infrastructure must be protected in accordance with international humanitarian law at all times,” UN Spokesperson Stéphane Dujarric told journalists in New York.

Escalating violence

In a statement, Mr. Pedersen said he was gravely concerned by the uptick in violence in Syria, noting that there was pro-Government shelling and rocket fire into Idlib following the attack, with reports of civilian casualties and strikes on civilian infrastructure.

He also pointed to weeks of significant escalation in the northwest, where pro-Government airstrikes and attacks by the extremist group Hayat Tahrir al-Sham have occurred.

Turkish strikes were also reported in the northeast on Thursday, which came on the back of an attack against Turkish Government facilities, claimed by the Kurdish militant organization PKK.

Exercise 'utmost restraint'

Mr. Pedersen deplored the loss of life on all sides. “I urgently appeal to all sides to exercise the utmost restraint,” he said.

“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, in line with Security Council resolution 2254 (2015).”

The resolution endorses a road map for a peace process in Syria.

The UN envoy urged all sides in the conflict to ensure the protection of civilians and civilian infrastructure, in line with their obligations under international law.

“Today’s developments further highlight that the status quo in Syria is unsustainable and that, in the absence of a meaningful political path to implementing Security Council resolution 2254 (2015), I fear we will only see further deterioration, including in the security situation,” he said.