Israel attacked a UN-run school housing refugees in Gaza despite warnings that civilians were there, the UN has said.

UN spokesman Chris Gunness said "the world stands disgraced" by the attack, in which 15 died and dozens were hurt.

The Israeli military said an initial inquiry suggested soldiers responded to mortar fire. It says it is now holding a partial, four-hour humanitarian ceasefire.

Some 1,200 Palestinians and 55 Israelis have been killed in the conflict.

Most of the Palestinian deaths have been civilians.

Fifty-three Israeli soldiers have been killed along with two civilians. A Thai worker in Israel has also died.

Israel launched its offensive in Gaza after a surge in rocket fire from the territory.

Hamas, which controls Gaza, says it will not stop fighting until the blockade, maintained by both Israel and Egypt, is lifted.

The current conflict is now the longest between Israel and militants from Gaza.

A 2012 offensive lasted for eight days, and the 2008 conflict went on for 22 days.