U.N. Secretary-General Ban Ki-moon called on Thursday for an immediate investigation of an attack on a school in Syria's Idlib province that the U.N.'s children's agency is calling 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, but divergent death tolls are not uncommon in a conflict-torn Syria that has been largely inaccessible to international media for over two years.
UNICEF and the Syrian Civil Defense said the death toll is likely to rise as rescue efforts continue.
Juliette Touma, regional UNICEF chief of communication, said Wednesday's attack was the deadliest attack on a school in 2016, bringing the overall death toll of children killed in such attacks in 2016 to 54.
According to Touma, 591 children were killed in 2015 in Syria.
Prior to Wednesday's attack, the deadliest assault on a school was reported in April 2014 when 30 children were killed in airstrikes that hit a school in the rebel-held part of Aleppo city, according to UNICEF.
UNICEF said it has verified at least 38 attacks on schools this year across Syria, whether in government-held areas or rebel-controlled territory, compared to 60 attacks last year.
"In general there are one in three schools in Syria that can't be used anymore because they were damaged or destroyed or used for military purposes or sheltering the displaced," Touma told The Associated Press, speaking from Amman, Jordan.
On Thursday, Syria's state TV said two students were killed and 13 others were wounded by projectiles fired by rebel fighters at a school in the government-held western part of Aleppo.
Elsewhere, at least eight people were killed in government shelling of Douma, a rebel-held suburb east of the capital Damascus, according to the Syrian Civil Defense team and the Observatory. The first responders said that there was a child among the eight who died.
UNICEF says over 1.7 million Syrian children remain out of school in 2016, a staggering figure but a drop from 2014 when 2.1 million were recorded as not attending classes. The U.N. agency says another 1.3 million are at risk of dropping out this year. In the rebel-held part of Aleppo, teachers and volunteers have set up underground schools to ensure some classes continue amid a punishing bombing campaign and a siege that has tightened since July.
Russian President Vladimir Putin meanwhile defended his support for the Syrian army's siege of the rebel-held eastern districts of the city of Aleppo, saying it is necessary to crush the militants there.
Putin spoke on Thursday to international foreign policy experts in Sochi. He said the choice was between "keeping a terrorist nest there or crushing that nest while minimizing civilian casualties."
Russia's air campaign in support of the Syrian troops' offensive on Aleppo has drawn international outrage. Moscow has denied striking civilians and has blamed Washington for failing to persuade the U.S.-backed rebels in the city to cut their ties with fighters from al-Qaida's branch.
Putin shrugged off calls to end the onslaught on Aleppo, saying the U.S.-backed offensive for Islamic State-held Mosul in Iraq would have to be halted, under the same logic.
Airstrikes by Russian and Syrian government planes on Aleppo have been halted for nine days now in expectations of evacuations, but efforts have failed because Syrian rebels say there have been no safety guarantees for the evacuees. The rebels also say Russia and the government are not allowing aid into the besieged, eastern rebel-held districts of Aleppo that are home to some 275,000 people.
A government ground offensive attempting to push into the rebel-held part of the city, and airstrikes in rural Aleppo have continued.
In Geneva, U.N. official Jan Egeland, said efforts would be renewed to secure the evacuation of nearly 200 wounded from eastern Aleppo districts, and allow medical and food supplies into the besieged part of the city.
He said the Syrian government had denied humanitarian access to eastern Aleppo as part of a monthly U.N. plan to access 25 besieged and remote areas in Syria.
"We need to overturn that decision because east Aleppo needs humanitarian supplies, they need it urgently," Egeland said. "If not ... it will be the worst winter in now the six winters we have had in the conflict."
An Iranian news website said a senior commander from Iran's paramilitary Revolutionary Guard was killed in Syria.
Thursday's report on Mizanonline.ir says Gen. Gholamreza Samei, whose service dates back to the Iran-Iraq war of the 1980s, was killed in Syria on Wednesday. Samei was also formerly posted to a remote area in east of Iran.
The report says he was in an advisory assignment in Syria. It did not elaborate.
Iran is a main ally of Syrian President Bashar Assad.