22 children killed in air raid on school - UNICEF

"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 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.

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.

Asked about the attack, Russia's UN ambassador Vitaly Churkin responded: "It's horrible, horrible. I hope we were not involved."

Syrian government forces and their Russian ally have been accused by rights groups of carrying out indiscriminate attacks on civilian infrastructure.

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 told AFP.

"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.

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.

'Intentional' attack

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 rebel groups and jihadists 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.

Russia is facing pressure at the United Nations to rein in its Syrian ally and halt air raids in rebel-held east Aleppo, where 250,000 civilians have been living under siege since July.

UN aid chief Stephen O'Brien said he was "incandescent with rage" over the Security Council's failure to take action, deploring that "nothing is actually happening to stop the war, stop the suffering."

With food growing scarce, "civilians are being bombed by Syrian and Russian forces, and if they survive that, they will starve tomorrow," said O'Brien.

"Aleppo has essentially become a kill zone."

Russian ambassador Churkin shot back, accusing O'Brien of making "arrogant remarks" and failing to recognise that Russia had declared a humanitarian pause that he maintained had been holding for eight days.

"If we needed to be preached to, we would go to a church," Churkin quipped.

The ambassador blamed opposition rebels and Al-Qaeda-linked jihadists for the failure at the weekend of a UN plan to evacuate the wounded from Aleppo and charged that the UN official was not objectively presenting the facts.

US ambassador Samantha Power criticised Russia, saying it had never worked cooperatively with the United Nations during the pauses to ensure humanitarian relief.

"You don't get congratulations and credit for not committing war crimes for a day or a week," said Power.

The bitter exchange came as the latest attempt to revive a ceasefire fell flat.

Meanwhile, US Defense Secretary Ashton Carter and his British counterpart Michael Fallon said an offensive to drive IS out of its Syrian stronghold of Raqa, 160 kilometres (100 miles) east of Aleppo, 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 jihadists' main Iraqi bastion of Mosul.

Syria's conflict erupted in March 2011 with protests calling for the ouster of President Bashar al-Assad.

But it has since evolved into a multi-front war, pitting jihadists, rebels, government forces and Kurdish militia against each other.

More than 300,000 people have been killed and millions forced to flee their homes.