At least 40 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.

There was no immediate response from the Israeli forces but they said they are looking in to the reports.

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

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.

In Tuesday's fighting at least 22 people, including four Israeli soldiers, were killed and 50 others injured. All the four Israeli soldiers, all from elite Golan Brigade were killed in 'friendly fire' when a tank, deployed as part of operation Cast Lead, accidentally fired a live round at their positions in an abandoned building in the Jabalya refugee camp.

So far a total of five Israeli soldiers have been killed in the conflict and a military spokesman in Tel Aviv said that among the Palestinian killed were 130 armed Hamas fighters.