At least 42 people were killed and scores injured in an Israeli strike on a UN school in Gaza strip, the fourth such attack on Tuesday, Palestinian medics said.

With this attack, the death toll since the launch of the offensive on December 27 rose to over 620.

Hundreds of Palestinian refugees had taken shelter at the school located in the northern town of Jabaliya when an Israeli tank shelled the building, they said.

People inside the building had taken refuge from the raging conflict in the territory. The toll quickly rose as rescuers struggled through the rubble.

Meanwhile, Israel Defence Forces said an initial inquiry by its forces operating in the area has indicated that a number of mortar shells were fired at them from the school.

'In response to incoming enemy fire, the forces returned mortar fire to the source. This is not the first time that Hamas has fired mortars and rockets from schools, in such a way deliberately using civilians as human shields in their acts of terror,' the army said in a statement.

Earlier in the day, Israeli air strikes had hit three UN-run schools in which 10 people were killed.

Israeli troops backed by tanks and helicopter gunships battled Hamas militants in all major Gaza towns as the country continued to reject appeals to halt the war.

The Israelis widened their offensive arc when its tanks rolled into the city of Khan Younis in southern Gaza Strip.