(Adds Israeli military comment)
By Nidal al-Mughrabi
GAZA, July 24 (Reuters) - At least 15 people were killed and many wounded on Thursday when Israeli forces shelled a U.N.-run school sheltering Palestinians in northern Gaza, said a spokesman for the Gaza health ministry, Ashraf al-Qidra.
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 his Twitter page.
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 on Thursday, Gunness told Reuters that Israeli forces had bombed U.N. shelters on three separate occasions since Monday, in incidents which did not cause injuries. (Additional reporting by Finbarr O'Reilly; Writing by Noah Browning; Editing by Gareth Jones and David Stamp)