The International Committee of the Red Cross accused Israel of failing in its international obligations after its staff were met with "shocking" scenes.
One medical team found 12 bodies in a shelled house, and alongside them four very young children, too weak to stand, waiting by their dead mothers, the ICRC said.
Aid workers had been denied access to the site for days, it added.
"This is a shocking incident," Pierre Wettach, ICRC head for Israel and the Palestinian territories said in a statement.
"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."
Hamas is hiding behind civilians and even using UN facilities to carry out its attacks. Most of the civilian casualties in this conflict can still be blamed on Hamas. However, this situation is now a major humanitarian disaster, and all sides must do what they can to both prevent civilian casualties and allow aid groups access to wounded civilians. If the Red Cross, long a neutral and objective voice in world conflicts, is criticizing Israel this harshly, then Israel really needs to respond.
I'm extremely worried that this situation is escalating. There are new reports of rocket fire from Lebanon. Hizbullah appears to be adding its voice to the situation...