An Israeli helicopter strikes

A number of large explosions have been reported in Gaza City following a fourth day of Israel's ground assault.

Palestinian sources have said at least 40 people have been killed in an Israeli air strike on a United Nations-run school.

Earlier in the day, three Israeli soldiers were killed and several wounded after being shelled by "friendly fire".

These pictures show a missile being dropped on Gaza City and the aftermath of the attacks in its hospitals and destroyed buildings.