More than 100 members of one extended Gazan family were allegedly herded by Israeli soldiers into a house that was subsequently shelled, killing up to 30 people and leaving others wounded and living with the corpses for three days.

The alleged atrocity, denied by Israel, was one of several for which the United Nations High Commissioner for Human Rights yesterday demanded “credible and independent” investigations, and raised the possibility of Israeli leaders being prosecuted for war crimes. Though her comments were addressed at both Israel and Hamas, Navi Pillay said that scores of people, including children, had been killed or wounded in “Israel's totally unacceptable strikes”.

“Violations of international humanitarian law may constitute war crimes for which individual criminal responsibility may be invoked,” she added.

Hamas has stepped up efforts to eliminate Israeli informers, killing more than half a dozen since the start of Operation Cast Lead 14 days ago. It has also cracked down on rival Fatah members, placing hundreds under house arrest and killing or maiming others, according to Israeli media.

Ms Pillay's remarks came as the UN Office for the Co-ordinator of Humanitarian Affairs released a report on what it described as one of the gravest incidents of the conflict. Citing “several testimonies” without identifying them, it accused Israeli soldiers of ordering about 110 Palestinians, half of them children, into a single house in Zeitun, a district of Gaza City, and warning them not to leave.

“Twenty-four hours later, Israeli forces shelled the home repeatedly, killing approximately 30,” the report said. “Those who survived and were able walked two kilometres to Salah Ed Din road before being transported to the hospital in civilian vehicles. Three children, the youngest of whom was five months old, died upon arrival at the hospital.”

The Israel Defence Forces (IDF) denied herding the Palestinians into the building, or firing on any building in that neighbourhood on the date in question. “Both parts are denied by us,” said Ilan Tor, a spokesman, cautioning against Hamas propaganda.

Survivors, rescuers and medics contacted by The Times painted a harrowing picture of what allegedly happened, though some of the details varied. Arafat Helmi Samouni, 36, was in Shifa hospital, Gaza City, seeking to recover the bodies of three brothers and other closer relatives, eleven in all. He said that roughly 400 members of his extended family lived in about 40 houses in a neighbourhood. As he told it, Israeli soldiers arrived last weekend, ordered the inhabitants out of their homes and herded them into a house near the mosque. The soldiers gave no reasons for their actions. Two or three hours later, he said, they opened fire on the house. Some tried to escape and were shot. The Israelis then shelled the house. “Everyone started losing consciousness and the bodies started piling up. We didn't know who died and who was wounded. We started running out with our hands raised. The Jewish started shooting but not directly at us,” he said. Around half escaped. Mr Samouni ran to an uncle's house 200 yards away where he found a cousin, Nedal, 32, lying in the doorway, shot in both legs. “He bled for seven hours and died in my arms,” he said. Mr Samouni stayed in the house for two more days with no food or water before slipping away through a neighbourhood that he said had been flattened.

Once safe he tried to call a brother's house. His niece, Apia, 8, answered. “She said, ‘Oh, uncle, we want water and food. My father and mother are dead. Everyone around me is dead except my little brothers'. She said they were hiding behind a closet.”

Abed el-Aziz Abu Aisha, one of a Palestinian Red Crescent Society team allowed into the neighbourhood by Israel on Wednesday, said that he would never forget what they found in the house - desperate and emaciated children, wounded survivors and the corpses of those shot or crushed.

At the Shifa hospital Mads Gilbert, a Norwegian doctor, said that he had amputated the wounded left hand of a nine-month-old baby because it had festered for so long

Another girl, Amal Atteia Samouni, 11, was severely dehydrated and in shock after being partially buried under rubble. “The girl's 22-year-old brother, who was at their bedside, described how they had been buried in the house for days, moving around the dead bodies,” Dr Gilbert said.

Ahmed Ibrahim Samouni, 13, who was wounded in the leg and chest, reeled off a list of dead relatives: “Abu Salah died, his wife died,” he told Reuters. “Abu Tawfiq died, his son died, his wife also died. Mohammed Ibrahim died, and his mother died. Ishaq died and Nasar died. The wife of Nael Samouni died. Many people died.”