Gaza is on "a knife-edge" and the Middle East is facing a serious challenge, UN Secretary General Ban Ki-moon said.
Ban spent Wednesday talking with world leaders including Israeli Prime Minister Benjamin Netanyahu, Palestinian president Mahmoud Abbas and Egypt's President Abdel Fattah el-Sisi, as well as US Secretary of State John Kerry. The talks took place as Israeli warplanes pounded Gaza, killing dozens, in a major new confrontation with Palestinian militants, as Hamas showed its firepower and sent thousands running for shelters across the country.
Ban said: "I am alarmed by the new wave of violence that has engulfed Gaza, southern Israel and the West Bank, including East Jerusalem. This is one of the most critical tests the region has faced in recent years.
"Gaza is on a knife-edge. The deteriorating situation is leading to a downward spiral which could quickly get beyond anyone's control.
"The risk of violence expanding further still is real. Gaza, and the region as a whole, cannot afford another full-blown war. "
Ban condemned the rocket attacks launched from Gaza on Israel, saying: "Such attacks are unacceptable and must stop."
He said he had also urged Netanyahu to exercise maximum restraint and to respect international obligations to protect civilians.