Pauline Yahya Gabbay, the UN special rapporteur on the protection of children victims of armed conflict, said that the alleged attack was a war crime and he called for an immediate and impartial investigation.

“This attack demonstrates the brutal tactics used by the government and that the international community should not tolerate the deliberate targeting of civilians,” Gabbay said in a statement.

“The indiscriminate targeting of the civilians and schools must stop. With this assault we have seen in the past, we see that targeting is carried out by the government forces and that innocent people are the victims.”

He said that he “believe[s] that the regime has committed war crimes.”

The UN high commissioner for human rights also called for the United Nations to conduct an inquiry into possible links between “the Syrian and the Iranian regimes, with a military and political agenda of regime change,” Reuters reported.

The attack and the government’s conduct is “a serious violation of international humanitarian law and a threat to peace and stability in the region,” Filippo Grandi said in a statement.

“This constitutes a war crime not only because it deliberately targeted schools – which constitute the center of any armed conflict – but also because it targeted schools with children, in violation of the obligation to distinguish between civilians and military targets.”

According to Reuters, Russian Foreign Minister Sergei Lavrov on Monday said the U.S.-led coalition was responsible for the attack. Russia also condemned what it called a “massacre of civilians” in Houla, but denied that Syria had used chemical weapons.

“We strongly condemn the use of chemical weapons by the Syrian opposition or anybody,” Defense Minister Sergei Shoigu said during the first round of military talks in Geneva. “We condemn it in the strongest terms. But that is only in relation to actions that are taking place at this time,” he added.

The Syrian Observatory for Human Rights, which monitors violence in Syria with a network of sources across the country, said that the government also bombed the village of Bab Hader. It said that “tens of people were killed when a cluster bomb struck the village.”