UN Secretary-General Ban Ki-moon said on Thursday he was appalled by an attack on a UN-run school in the northern Gaza strip that had killed women, 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.
The IDF was looking into the matter.