UN Secretary General Ban Ki-moon has said the attack that killed 10 people at a UN-run Gaza school was a "moral outrage and a criminal act".
He also stated that the strike constituted a "gross violation of international law", BBC reported.
The US was appalled by the "disgraceful" shelling in Gaza school, US State Department spokeswoman Jen Psaki said.
Israel has declared a "humanitarian ceasefire" in most of Gaza for seven hours on Monday.
Brig Gen Yoav Mordechai said the ceasefire would apply to all of the Gaza Strip except eastern Rafah from 10am (07:00 GMT) to 19pm.
Sunday's blast hit the entrance of the school, which is sheltering thousands displaced by the conflict. Israel denies firing into the school.
The Israeli military said it had targeted three militants from the Islamic Jihad group on a motorbike near the school, though the group did not report any of it members killed or injured.
Gaza health officials say 30 people have died on Sunday, while militants continue to fire rockets into Israel.
The UN has warned that a "health disaster of widespread proportions" is unfolding in Gaza, with medical services facing collapse.
