"The situation in Gaza has reached a dangerous new low," Guterres said ahead of a Security Council meeting on the volatile situation.

"We need immediate humanitarian access throughout Gaza, so that we can get fuel, food and water to everyone in need," he said, adding that the blockaded enclave's health system was on the brink of collapse.

Guterres' call for the protection of civilians came as Israeli Prime Minister Benjamin Netanyahu said his country's fierce bombardment of Gaza was "just the beginning" of the response to Hamas attacks launched on Saturday that left 1,300 -- mostly civilians -- dead.

Hamas gunmen also abducted around 150 Israelis and foreigners, taking them to Gaza as hostages.

Nearly 1,800 Gazans -- again most of them civilians and including over 580 children -- have been killed in the waves of missile strikes on the densely populated enclave, the health ministry said.

"Even wars have rules," Guterres said. "International humanitarian law and human rights law must be respected and upheld; civilians must be protected and also never used as shields.

"It is imperative that all parties -- and those with influence over them -- do everything possible to achieve these steps."

Israel has given more than one million Palestinians 24 hours to leave the northern Gaza Strip ahead of an expected ground offensive -- an exodus that Guterres said would set off a "humanitarian catastrophe."