In the wake of the criticism over civilian casualties, Israel agreed to set up a “humanitarian corridor'' to ship vital supplies into the Gaza Strip. Under the plan, put forward by the Israeli Defense Ministry, Israel would suspend attacks in certain areas to allow people to get supplies.

At UN headquarters, Secretary-General Ban Ki-moon called the Israeli bombardment of UN facilities in Gaza ``totally unacceptable.'' Israel's shells have fallen around three schools, including the girls school hit Tuesday, and a health center for Palestinian refugees.

Ban added that it was “equally unacceptable'' for militants to take actions that endanger Palestinian civilians, referring to the practice of militants making attacks from residential areas.

In Gaza, UN official Christopher Gunness hoped an investigation would show whether militants were using UN schools for weapons or activities. ''It is just not in our interests to have militants, whether in war or peace, in our installations,'' he said in a statement.

Some 15,000 Palestinians have packed the UN's 23 Gaza schools because their homes were destroyed or to flee the violence. The UN provided the Israeli military with GPS coordinates for all of them.

The three mortar shells that crashed down on the perimeter of the UN school struck at mid-afternoon, when many people in the densely populated camp were outside getting some fresh air, thinking an area around a school was safe.

Images recorded by a cameraman from AP Television News showed crowds fleeing the scene, pavements smeared with blood and battered bodies being carried off by medics and bystanders. A youth who limped away was helped along by several others. Sandals lay scattered on the pavement by a pock-marked wall.

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