UN Secretary-General Ban Ki-moon said he is "appalled" by an attack on a school in Gaza which has killed at least 16 people.

The UN-run school was sheltering Palestinians at the time of the shelling, which Ban blamed on a "massive Israeli assault".

"Circumstances are still unclear. I strongly condemn this act. Many have been killed - including women and children, as well as U.N. staff," Ban said in a statement.

"Throughout the day, our staff had been attempting to arrange a humanitarian pause in the hostilities so that civilians could be evacuated.

"I express my profound condolences to the families of the victims and those of so many hundreds of innocent Gazans who have tragically been killed as a result of the massive Israeli assault."