BEIRUT — U.N. Secretary-General Ban Ki-moon called Thursday for an immediate investigation of an attack on a school in Syria’s Idlib province that the U.N.’s children’s agency says is one of the deadliest of its kind in the country’s six-year war.

UNICEF raised its toll for Wednesday’s attack to 28 dead, among them 22 children and six teachers.

Ban said in a statement that the attack, carried out against rebel-held territory, may amount to a war crime if found to be deliberate.

“If such horrific acts persist despite global outrage, it is largely because their authors, whether in corridors of power or in insurgent redoubts, do not fear justice. They must be proved wrong,” he said.

Witnesses said overhead jets targeted the school in the town of Hass as many as 10 times, around midday Wednesday. Opposition activists blamed the strikes on Russian and government planes.

Idlib is the main Syrian opposition stronghold, though radical militant groups also have a large presence there. It has regularly been hit by Syrian and Russian warplanes as well as the U.S.-led coalition targeting Islamic State militants.

The Russian military denied responsibility, calling the accusations a “sham.”

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said a Russian drone flew over the school building on Thursday and found its roof intact and no bomb craters around it.

Russian warplanes didn’t fly over the area the previous day, Konashenkov said.

A team of first responders, the Syrian Civil Defense, said on Thursday that the airstrikes killed at least 35 people, mostly children.

The Britain-based Syrian Observatory for Human Rights said 15 students were killed, as well as four teachers and three other women. It was not immediately possible to reconcile the separate figures.