BEIRUT » The UN’s children’s agency today raised the death toll from a brutal attack the previous day on a school in Syria’s rebel-held Idlib province to 28, including 22 children and six teachers, and suggested it may have been the deadliest attack on a school in the country’s civil war.

The airstrikes struck the village of Hass around midday Wednesday, hitting a residential compound that houses a school complex. The Syrian Civil Defense first responder team and the Britain-based Syrian Observatory for Human Rights said today that the airstrikes killed at least 35 people, mostly children.

The Observatory 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. They said that two schools in the area were hit with 11 airstrikes around midday.

UNICEF Executive Director Anthony Lake called the airstrikes an “outrage,” adding that if found to be deliberate the attacks would be considered a war crime.

“This latest atrocity may be the deadliest attack on a school since the war began,” Lake said in a statement. “When will the world’s revulsion at such barbarity be matched by insistence that this must stop?”

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. An activist at the scene said as many as 10 airstrikes were believed to have hit Hass on Wednesday.

Juliette Touma, regional UNICEF chief of communication, said Wednesday’s attack was the deadliest attack on a school in 2016, bring 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.

Syria’s state TV today 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 Doumas, 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.

In other developments today, U.N. official Jan Egeland, speaking in Geneva, said efforts will 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.

Airstrikes by Russian and Syrian government planes on Aleppo have been halted for nine days now in expectations of the 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.

Egeland said a lack of trust, fear, and misunderstandings, as well as unacceptable preconditions, have prevented evacuations.

“We are not giving up,” Egeland said.

However, he said the Syrian government has 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.”