Air strikes on Syria's opposition-held Idlib province hit a school and the surrounding area on Wednesday, killing 22 children and six teachers, the UN children's agency UNICEF said Wednesday.
"This is a tragedy. It is an outrage. And if deliberate, it is a war crime," said UNICEF director Anthony Lake.
The Syrian Observatory for Human Rights (SOHR) earlier said "warplanes — either Russia or Syrian — carried out six strikes" in the village of Hass, including on a school complex killing at least 35 civilians, including 11 schoolchildren.
The White Helmets civil defence group released pictures of four rescue workers clambering over a mound of rubble in search of survivors after what it said was a "double-tap" strike on the school.
The raids hit Hass around 11:30 am (0830 GMT), an activist with the opposition Idlib Media Centre said.
"One rocket hit the entrance of the school as students were leaving to go home, after the school administration decided to end classes for the day because of the raids," the activist said, speaking on condition of anonymity.
Lake said the school compound was "repeatedly attacked," adding that it may be the deadliest attack on a school since the war began more than five years ago.
"When will the world's revulsion at such barbarity be matched by insistence that this must stop?" added the UNICEF director.
Other activists from the province circulated a photograph on social media of a child's arm, seared off above the elbow, still clutching the strap of a dusty black rucksack.
Shaky video footage depicted rescue workers sprinting towards the site of the raids and pulling a frail, elderly man out of a collapsed building.
The authenticity of the pictures and footage could not be independently verified.
The latest attacks took to 89 the number of civilians killed in air strikes on Idlib province in the past seven days, said the Observatory.
Asked about the attack, Russia's UN ambassador Vitaly Churkin responded: "It's horrible, horrible. I hope we were not involved."
"It's the easiest thing for me to say no, but I'm a responsible person, so I need to see what my Ministry of Defence is going to say."
Syrian regime forces and their Russian ally have been accused by rights groups of carrying out indiscriminate attacks on civilian infrastructure.
A leading opposition group condemned the raids.
The Istanbul-based National Coalition said Russian and regime warplanes "targeted children in their schools, deliberately and intentionally hitting civilians with high-explosive material".
Idlib province is controlled by the Army of Conquest, an alliance of opposition groups and militants including the Fateh al-Sham Front, which changed its name from Al-Nusra Front after breaking off ties with Al Qaeda earlier this year.
Syrian and Russian warplanes regularly bomb Idlib, but air strikes have intensified in recent weeks, according to the Observatory.
Swathes of eastern Syria is controlled by Daesh.
On Wednesday, US Defence Secretary Ashton Carter and his British counterpart Michael Fallon said an offensive to drive Daesh out of its Syrian stronghold of Raqqah would begin in the next few weeks.
The US-led coalition is currently supporting a 10-day-old assault by Iraqi and Kurdish forces on the terrorist group's main Iraqi bastion of Mosul.
Syria's conflict erupted in March 2011 with protests calling for the ouster of regime leader Bashar al Assad.
The civil war pits Bashar al Assad, backed by Russia, Iran and Shia Muslim militias from Lebanon, Iraq and Afghanistan against an array of mostly Sunni Muslim opposition groups, including some backed by Turkey, Gulf monarchies and the United States.
More than 300,000 people have been killed and millions forced to flee their homes.