Israeli shells slammed into a compound housing a U.N. school in the Gaza Strip on Thursday, killing at least 15 people and wounding scores more who were seeking shelter from the bloody fighting across the region.

At least 200 were wounded in the attack on the compound in Beit Hanoun in northern Gaza, according to health ministry official Ashraf al-Kidra. Reports from the scene found pools of blood staining the school courtyard amid scattered books and belongings.

The U.N. refugee agency, UNRWA, said it had been trying to arrange a window in the fighting between the Israel Defense Forces and Hamas militants for civilians to leave the compound.

I will never forget the utter shock on these kids faces - they thought they were safe in a UN school #Gaza pic.twitter.com/0YRHVc6pxz - Dan Rivers (@danieljerivers) July 24, 2014

"Over the course of the day, UNRWA tried to co-ordinate with the Israeli Army a window for civilians to leave and it was never granted," spokesman Chris Gunness said on his Twitter feed. "Precise co-ordinates of the UNRWA shelter in Beit Hanoun had been formally given to the Israeli army."

Israel’s IDF said it was “reviewing” the incident but said militant rockets had earlier been fired from the compound. It also said via it's Twitter account that it had advised the Red Cross to evacuate the compound, but that "Hamas prevented civilians from evacuating the area during the window that we gave them."

Last night, we told Red Cross to evacuate civilians from UNRWA's shelter in Beit Hanoun btw 10 am & 2 pm. UNRWA & Red Cross got the message. — IDF (@IDFSpokesperson) July 24, 2014

The U.S. State Department, in a statement, said the incident "underscores the need to end the violence and to achieve a sustainable ceasefire and enduring resolution to the crisis in Gaza as soon as possible."

"UN facilities in Gaza are sheltering more than 140,000 Palestinians, including many innocent children, and must remain safe, neutral sanctuaries for fleeing civilians. We call on all parties to protect these facilities from the conflict and we have condemn those responsible for hiding weapons in United Nations facilities in Gaza. We urge all parties to respect civilian life and comply with international humanitarian law."