Airstrikes on Wednesday killed at least 28 people, mostly children, when warplanes targeted a school in Syria’s Hass village in Idlib province, according to the United Nations Children’s Fund. The U.N. organization described the attack as possibly the deadliest on a school since the country plunged into war.

The airstrikes, which occurred around midday, killed about 22 children and six teachers. Unicef Executive Director Anthony Lake called the attack “an outrage.”

"This is a tragedy. It is an outrage. And if deliberate, it is a war crime. This latest atrocity may be the deadliest attack on a school since the war began more than five years ago,” Lake said in a statement. “When will the world’s revulsion at such barbarity be matched by insistence that this must stop?”

News of the airstrikes follows a scathing speech by the U.N. Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator, Stephen O’Brien, to the U.N. Security Council.

In his monthly address to the 15-member council Wednesday on the worsening situation in war-torn Syria, O’Brien said, “If you don’t take action, there will be no Syrian peoples or Syria to save – that will be this Council’s legacy, our generation’s shame. It is in your hands today to take the right path, and avert this looming irreversible tragedy of our time.”

He blamed the near-destruction of Syria’s Aleppo city on the country’s President Bashar Assad and Russia’s involvement in the ongoing war, which has entered its sixth year.

“Aleppo has essentially become a kill zone. Since my last report to this Council less than a month ago - 400 more people have been killed and nearly 2,000 injured in eastern Aleppo. So many of them – too many of them – were children,” O’Brien said.

Russia said Tuesday that Moscow-led warplanes hadn’t even approached Aleppo, let alone conduct airstrikes since Moscow announced a pause in hostilities last Tuesday. But rights groups said airstrikes had resumed in some areas Sunday.

O’Brien said Wednesday that Russian and Syrian warplanes dropped leaflets on eastern Aleppo that read, “This is your last hope. ... Save yourselves. If you do not leave these areas urgently, you will be annihilated.” The notes end with: “You know that everyone has given up on you. They left you alone to face your doom and nobody will give you any help.”

Eastern Aleppo, which remains under siege, has reportedly not received any U.N. assistance in four months. Food supplies are low as are medical and water stocks.