A United Nations report that Israel ordered civilians into a building and then shelled it marks yet more evidence of widespread targeting of civilians in the Gaza assault.

The UN report calls for a war crimes investigation into Israel's shelling of a civilian building in Gaza city Jan. 4. It described the shelling as "one of the gravest incidents" since Israeli infantry and armoured troops entered Gaza.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) cited eyewitnesses to report that on Jan. 4 Israeli ground troops ordered more than 100 Palestinian civilians into a house in Gaza city. Allegre Pachecho, deputy chief of OCHA, said that the following day the building was hit by Israeli shells.

Israeli human rights groups say at least 30 people were killed in the shelling.

Red Cross and Red Crescent rescue teams were prevented from reaching the building in the Zeitoun neighbourhood of Gaza city. But after intensive negotiations the team reached the area on foot, and were able to rescue starving toddlers found near the bodies of their mothers.

Israel's military bombardment of Gaza, codenamed Operation Cast Lead, has now killed nearly 800 Palestinians and wounded more than 3,000. Over 200 of the dead, and over a thousand of the injured are children.

Ten Israelis are reported to have been killed, most of them through friendly fire accidents. 

A number of UN schools have been shelled by Israeli troops. The most serious incident involved a UN Relief and Welfare Agency (UNRWA) school in the Jabaliya refugee camp on the outskirts of Gaza city. Forty-three civilians sheltering there lost their lives.

The Israeli Defence Forces (IDF) originally claimed that Palestinian gunmen had been firing from inside the clearly marked UN school. To back up its claims the IDF showed footage of gunmen firing from a UN facility.

On Friday, however, UNRWA spokesman Chris Gunness told the Israeli daily Haaretz that the army had conceded wrongdoing.

"In briefings senior IDF officers conducted for foreign diplomats, they admitted the shelling to which IDF forces in Jabaliya were responding did not originate from the school," Gunness said. "The IDF admitted in that briefing that the attack on the UN site was unintentional."

Gunness noted that all of the footage released by the IDF of militants firing from inside the school was from 2007. "There are no up-to-date photos," Gunness said. And in 2007, he said, "we abandoned the site and only then did the militants take it over."

Several years ago the Israelis claimed to have satellite pictures of an UNRWA ambulance being used by gunmen to transport missiles. Following an investigation, the missile turned out to be a folded stretcher, and the gunmen medics.

The Israelis were given the full GPS coordinates of all UN facilities in the Gaza strip prior to Israel's military operation.

John Ging, head of the UN in Gaza, said that though the IDF had acknowledged wrongdoing, the UN was ordering a full and independent investigation. "We monitor our facilities very carefully and we make sure that no combatants enter any of our buildings," Ging told IPS on phone from Gaza.

"This is an extremely serious matter for us. Any fighters entering our premises endanger the lives of the civilians sheltering there, and would compromise our ability to help civilians in the future," he added.

Following the shelling of a UN aid convoy on Thursday, which killed a driver and seriously injured another UN employee, Ging said that the UN was working at the highest level with the Israelis to ensure safe passage for future aid convoys.

The aid convoy had been hit during a daily three-hour ceasefire established by the IDF to enable humanitarian aid to enter the coastal territory. UNRWA temporarily suspended its aid relief following the incident.

"Prior to Thursday's incident we were repeatedly assured that our convoys would be safe and could enter Gaza," Ging told IPS. "The situation at present is completely unacceptable.

"What we need is a reliable mechanism that ensures coordination between the Israeli military commanders and their troops on the ground to avoid future incidents of this nature," said Ging.

According to one Palestinian media report, a family with 33 members has been trapped for six days in a single room of their house in the Al-Mughrqa area south of Gaza city.

Salem Yousel Al-Wahidi was quoted as saying that soldiers were occupying his house. He said they had refused the family food and water, and had also refused milk for the family's babies.

The family appealed to the International Committee of the Red Cross (ICRC) and other human rights organisations to intervene urgently.