At least 35 people, many of them schoolchildren, have been killed in air strikes on a rebel-held village in northwestern Syria.

The raids hit a residential area and a school in the village of Hass, in Idlib province. The U.N. said it may be “the deadliest attack on a school since the war began.”

A school complex was reportedly among several locations destroyed in the village of Haas, in Idlib-a province in northwestern Syria. Syrian state media quoted a military source as saying several “terrorists” had been killed when their positions were hit. The report did not mention a school.

The government says the village is controlled by two rebel factions-Al-Qaeda’s Syrian affiliate, Jabhat al-Nusra, and Ahrar Al-Sham. Anthony Lake, executive director for the U.N.’s children's charity UNICEF, said 22 children were killed in Hass. Lake called it an ‘atrocity’ and possibly ‘the deadliest attack on a school since the war began more than five years ago.’”

It is unclear at this point whether it was the Syrian or the Russian air force that carried out the airstrikes. A spokeswoman for the Russian Foreign Ministry said Moscow is demanding an immediate investigation by international organizations. She said Russia had “nothing to do with the attack.”

“It’s a lie, Russia has nothing to do with this terrible tragedy, this attack. We demand maximum attention on this tragedy and an immediate investigation,” said Maria Zakharova, spokesperson of Russian Ministry of Foreign Affairs.

A Syrian government source also denied targeting schools. The source told CCTV, “We put a lot of scrutiny into any air strikes that could hit schools as collateral damage.”

About an hour’s drive north of Hass, rebels shells Aleppo city hitting government-controlled western parts of the city. A school was hit. Government news agency SANA reports three children dead in that attack, and 14 wounded.