An attack that killed 10 people at a UN-run Gaza school was a "moral outrage and a criminal act", UN Secretary General Ban Ki-moon has said.
It constituted a "gross violation of international law", Mr Ban said.
State Department spokeswoman Jen Psaki said the US was appalled by the "disgraceful" shelling.
The blast hit the entrance of the school, which is sheltering thousands displaced by the conflict. Israel denies firing into the school.