The United Nations has dismissed Israeli claims that its bloody attacks on schools were directed at killing Palestinians fighters.

Christopher Gunness, spokesman for the UN refugee agency for Palestinian refugees (UNRWA) said with high confidence on Wednesday that there were no combatants or combative activities in the schools and school compounds targeted by Israel on Tuesday.

"We are calling for an independent investigation to establish the facts," he said, calling for those who broke the rules of war to be brought to justice.

Israeli forces attacked three UN-run schools in the Gaza Strip, killing at least 45 civilians, most of whom were Palestinians who had taken refuge in the buildings to escape the raining Israeli fire on Gaza.

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Israeli officials had tried to justify the incident as an attack on a militant stronghold.

Israeli ground forces began a full-scale invasion of Gaza on Saturday after pounding the region with airstrikes for a week.

According to Tel Aviv, the war on Gaza is aimed at ending rocket attacks against Israeli settlers, toppling Hamas and preventing the resistance group from rearming.

Hamas, however, demands a cessation of Israeli attacks and the opening of the Gaza border -- which has been closed due to the 18-month blockade imposed on the strip by Tel Aviv.

So far the Israeli aggression has killed over 660 Palestinians, half of whom are women and children. More than 2,950 others have been wounded and many are feared to be under the rubble.