Palestinians run for cover after a strike near the Al Shifa hopsital in Gaza City. AFP

One of the schools affected was in Jabalia refugee camp "after two days of heavy bombardments in the area," an UNRWA statement said, while the others were in Shati, or beach camp, and two in Bureij.

The health ministry in the Hamas-run Gaza Strip said on Thursday at least 27 people were killed in an Israeli strike near a UN-run school in the Jabalia refugee camp. "The bodies of 27 martyrs were recovered and a large number of wounded," ministry spokesman Ashraf Al Qudra said.

AFP footage showed several casualties as crowds of people rushed to help them, while it was not possible to independently verify the death toll. "Children under 10 were simply buying from the canteen and were cut into pieces," cried a woman in the school yard.

The UN agency for Palestinian refugees confirmed that one of its schools in Jabalia was damaged "after two days of heavy bombardments in the area."

UNRWA said that in the past day four of the agency's schools were damaged, while hosting nearly 20,000 displaced people combined. One of those affected was in Shati, or beach camp, while two others were in Bureij camp. Gaza's civil defence authority said at least 15 people were killed in Israeli strikes on Bureij on Thursday.

"The planes struck the whole neighbourhood, they pulled me out from under the rubble," said Bureij resident Hanan Abdulhadi, 50, blood running down her face.

Carrying his wounded two-year-old son, Mohammed Karajeh, 27, said "the whole house fell on us" with more than 20 homes destroyed.

Gaza's health ministry said 9,061 people have been killed in nearly four weeks of Israeli strikes and the recent ground assault.