The White House said it condemned the shelling of a UN school in the Gaza strip that was sheltering displaced Palestinians.
The US did not say Israel was responsible for the shelling, but officials in Gaza said Israel struck the school, killing 15 and wounding 90.
Israeli's military said it fired back after soldiers were targeted by mortar rounds launched from the vicinity of the school.
White House spokeswoman Bernadette Meehan also said the US is "extremely concerned" that thousands of Palestinians aren't safe in designated shelters, despite being told by Israel's military to leave their homes. She also condemned those hiding weapons in UN facilities.
All of these actions, and similar ones earlier in the conflict, are inconsistent with the UN's neutrality. This violence underscores the need to achieve a cease-fire as soon as possible.
There have been two days of quiet in Gaza. No rockets or missiles, no more destroyed houses, no more ambulance sirens screaming.
A three-year-old girl has been rescued from Gaza and brought to London for life-saving heart surgery.
A small team of NHS doctors and surgeons is travel to Gaza to set up facilities and establish how best to use specialists and equipment.