The United Nations says it has received reports that about 30 Palestinians were killed when Israeli forces shelled a house after they had moved about 110 civilians inside it.
"According to several testimonies, on 4 January Israeli foot soldiers evacuated approximately 110 Palestinians into a single-residence house in Zeitun, warning them to stay indoors," the UN report said on Friday.
"Twenty-four hours later, Israeli forces shelled the home repeatedly, killing approximately 30."
The incident took place on Monday in the Zeitun neighbourhood of Gaza City on Monday, the report said.
The UN Office for the Co-ordination of Humanitarian Affairs (OCHA) called it "one of the gravest incidents since the beginning of operations" by Israeli forces in Gaza on December 27.
"Those who survived and were able walked two kilometres to Salah Ed Din road before being transported to hospital in civilian vehicles," OCHA said.
"Three children, the youngest of whom was five months old, died upon arrival at the hospital."
The Israeli military said on Friday that it had no knowledge of the incident.
The Israeli B'Tselem human rights group quoted Meysa Fawzi al Samouni, a 19-year-old resident, as saying that soldiers forced her and dozens of others to move into the warehouse-like home of another resident before the attack.
"As far as I know, the dead and wounded who were under the ruins are still there," B'Tselem quoted her as saying.
"There was no water, no bread, nothing to eat"
13-year-old survivor of shelling
Ibrahim Samouni, a 13-year-old boy who was wounded in the leg and chest, told the Reuters news agency that he kept his three younger brothers alive and tried to help the injured adults lying among the dead after his mother was killed in the incident.
"There was no water, no bread, nothing to eat," he said.
"Abu Salah died, his wife died. 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."
Emergency workers from the International Committee of the Red Cross (ICRC) and the Palestinian Red Crescent said they were not able to reach the wounded for four days because Israel would not allow them safe passage.
They said the children were starving when rescuers finally reached them after the "unacceptable" delay.
"They were too weak to stand up on their own. One man was also found alive, too weak to stand up. In all, there were at least 12 corpses lying on mattresses," the ICRC said.
Pierre Wettach, the ICRC chief for Israel and the occupied Palestinian territories, said: "The Israeli military must have been aware of the situation but did not assist the wounded.
"Neither did they make it possible for us or the Palestinian Red Crescent to assist the wounded."
Israel has repreatedly insisted it has tried to avert civilian casualties during its two-week aerial and ground assault on the territory.
"From initial checking, we don't have knowledge of this incident. We started an inquiry but we still don't know about it," Avital Leibovich, an Israeli military spokeswoman, said about the reported shelling incident.
More than 780 people, including at least 257 children and 56 women, have been killed during Israel's aerial bombardment and ground offensive, the UN has said.