The president of the International Committee of the Red Cross on Saturday called on all sides to act to halt the "intolerable" human suffering of civilians in the Gaza Strip.
"This is a catastrophic failing that the world must not tolerate," said Mirjana Spoljaric, as Israel declared its war on Hamas had "entered a new phase" with its massive bombardment of Gaza.
"I am shocked by the intolerable level of human suffering and urge the parties to the conflict to de-escalate now," she said.
"The tragic loss of so many civilian lives is deplorable."
"It is unacceptable that civilians have no safe place to go in Gaza amid the massive bombardments, and with a military siege in place there is also no adequate humanitarian response currently possible."
Spoljaric was speaking hours after UN Secretary-General Antonio Guterres sharply criticised the "unprecedented escalation" of bombardments on Gaza, and called for an "immediate" ceasefire.
The Israeli army on Saturday warned residents of Gaza City, in the north of Gaza, that the area was now a "battlefield", ordering them to "evacuate immediately" to the south.
Israel unleashed an indiscriminate and relentless bombing campaign three weeks ago, shelling hospitals, residential buildings and other key infrastructure since October 7. At least 8,000 have been killed according to the Ministry of Health in Gaza, with almost half the victims being children.
Israel has threatened the besieged Strip with a ground invasion, and has ordered a complete blockade of the territory by cutting off water, food and fuel supplies from Gaza's 2.3 million inhabitants.
The UN High Commissioner for Human Rights Volker Turk warned on Saturday there was the potential for thousands more civilians to die if Israel presses a major ground offensive in Gaza.
The conflict is the fifth and deadliest in Gaza since Israel unilaterally withdrew troops and settlers from the Palestinian territory in 2005.