Ban Ki-moon, the UN secretary-general, says the deaths in Gaza "have shocked and shamed the world".
Ban described repeated attacks on UN facilities in Gaza as "outrageous, unacceptable and unjustifiable" and said Israel's actions raised new questions about international law and accountability.
Benjamin Netanyahu, the Israeli prime minister, said his country had the right to defend itself.
Al Jazeera's Nisreen El-Shamayleh reports from West Jerusalem.
Source: Al Jazeera