UN Secretary-General Ban Ki-moon says he's appalled by an attack on a United Nations-run school in the Gaza strip that killed civilians, including children, and UN staff.
"Circumstances are still unclear. I strongly condemn this act," Ban said in a statement. "Many have been killed - including women and children, as well as UN staff."
The Gaza health ministry said at least 15 people had been killed and some 200 wounded. Israel Radio, without citing a source, reported that most of those killed at the United Nations school were children.
"Throughout the day, our staff had been attempting to arrange a humanitarian pause in the hostilities so that civilians could be evacuated," said Ban.
More than 140,000 Palestinians have fled 17 days of fighting between Israel and Gaza militants, many of them seeking shelter in buildings run by the UN Refugee Works Agency (UNRWA). Israeli forces are trying to stop Hamas militants, which rules Gaza, and their allies from firing rockets into Israel.
Almost 750 people have been killed during the conflict in Gaza. Ban expressed condolences to the families of innocent civilians "killed as a result of the massive Israeli assault."
He once again "condemned Hamas rocket fire and called on Israel to exercise particular care to avoid any attack on United Nations premises where civilians have taken refuge."
Ban expressed alarm after rockets were found in a vacant UN school in Gaza for the second time in a week, warning in a statement that "those responsible are turning schools into potential military targets, and endangering the lives of innocent children."
A spokeswoman for US Secretary of State John Kerry said the deadly Gaza school incident "underscores the need" for a ceasefire and resolution of the conflict.
"This ... underscores the need to end the violence and to achieve a sustainable ceasefire and enduring resolution of the crisis in Gaza as soon as possible," said Jen Psaki, a spokeswoman for Kerry, who is trying to secure a truce to curtail 17 days of fighting.
"We again urge all parties to redouble their efforts to protect civilians," she said.
The director of a local hospital said various medical centres around Beit Hanoun in the coastal enclave were receiving the wounded. "Such a massacre requires more than one hospital to deal with it," said Ayman Hamdan of the Beit Hanoun hospital.
Israel said its troops were engaged in combat in the area with Hamas gunmen and it was investigating the incident at the school.
A Reuters photographer at the scene said pools of blood had collected on the ground and on student desks in the courtyard of the school near the apparent impact mark of the shell.
Scores of crying families who had been living in the school ran with their children to the hospital where the victims were being treated a few hundred metres away.
Laila Al-Shinbari, a woman who was at the school when it was shelled, told Reuters that families had gathered in the courtyard expecting to be evacuated shortly in a Red Cross convoy.
"All of us sat in one place when suddenly four shells landed on our heads ... Bodies were on the ground, (there was) blood and screams. My son is dead and all my relatives are wounded including my other kids," she wept.
Chris Gunness, spokesman for the main United Nations agency in Gaza, UNRWA, confirmed the strike and criticised Israel.
"Precise co-ordinates of the UNRWA shelter in Beit Hanoun had been formally given to the Israeli army ... Over the course of the day UNRWA tried to coordinate with the Israeli Army a window for civilians to leave and it was never granted," Gunness said on Twitter.
A statement from the Israeli military said soldiers were "in the midst of combat" with Hamas gunmen who fired rockets at the troops in the Beit Hanoun area.
Earlier, Gunness told Reuters that Israeli forces had bombed UN shelters on three separate occasions since Monday, in incidents which did not cause injuries.