At least 15 people have been killed and more than 200 injured when a UN-run school used as a shelter in Gaza was shelled, the Gaza health ministry says.

Hundreds of Palestinians were in the school in Beit Hanoun, fleeing heavy fighting in the area.

It is the fourth time that a UN facility has been hit in Israel's offensive against Hamas militants.

In the past 16 days of fighting, more than 725 Palestinians and 30 Israelis have been killed, officials say.

Israel launched its military offensive on 8 July with the declared objective of stopping Hamas firing rockets from Gaza.

Earlier on Thursday, UN humanitarian chief Valerie Amos said that it was "vital" to have a ceasefire.

"We have over 118,000 people now who are sheltering in UN schools... people are running out of food. Water is also a serious concern," she said.

She said the conflict meant 44% of Gaza was a no-go area for Palestinians, and residents were running out of food.

Israeli PM Benjamin Netanyahu said on Thursday he regretted each Palestinian civilian death, but said they were "the responsibility of Hamas".