The head of UN agency for Palestinian refugees issued a furious denunciation of an Israel strike on one of its schools in Gaza which killed 16 people on Wednesday.

"I condemn in the strongest possible terms this serious violation of international law by Israeli forces," said UNRWA commissioner General Pierre Krahenbuhl, saying the school's location had been communicated to the Israeli army 17 times.

"This is the sixth time that one of our schools has been struck," he said, indicating some 3,300 people had been sheltering in the school in Jabaliya refugee camp at the moment it was struck.

"We have visited the site and gathered evidence. We have analyzed fragments, examined craters and other damage. Our initial assessment is that it was Israeli artillery that hit our school, in which 3,300 people had sought refuge. We believe there were at least three impacts. It is too early to give a confirmed official death toll. But we know that there were multiple civilian deaths and injuries including of women and children and the UNRWA guard who was trying to protect the site. These are people who were instructed to leave their homes by the Israeli army.

The precise location of the Jabalia Elementary Girls School and the fact that it was housing thousands of internally displaced people was communicated to the Israeli army seventeen times, to ensure its protection; the last being at ten to nine last night, just hours before the fatal shelling." Krahenbuhl stated.