The ICRC normally conducts confidential negotiations with warring parties, and its accusation against Israel was a rare public criticism of one party in a conflict over a specific incident.
Eventually, rescuers from the international Red Cross and Palestine Red Crescent received permission to go into the shelled houses during the halt in fighting Wednesday, four days after the buildings were hit by Israeli shells.
"This is a shocking incident," Pierre Wettach, head of the ICRC for the region, said.
The rescue team "found four small children next to their dead mothers in one of the houses. They were too weak to stand up on their own. One man was also found alive, too weak to stand up," the statement said. "In all, there were at least 12 corpses lying on mattresses" in one of the houses, it added.
The organization said the children and the wounded had to be transported by donkey cart to ambulances.
"The Israeli military must have been aware of the situation but did not assist the wounded," the international Red Cross said. "Neither did they make it possible for us or the Palestine Red Crescent to assist the wounded."