"As humanitarian leaders, our position is clear: We will not participate in the establishment of any 'safe zone' in Gaza that is set up without the agreement of all the parties," they said in a joint statement.

"Under the prevalent conditions, proposals to unilaterally create 'safe zones' in Gaza risk creating harm for civilians, including large-scale loss of life, and must be rejected."

Israel has for more than a month been calling on the population in northern Gaza to evacuate south, as it pushes ahead with its air and ground invasion of the Palestinian territory.

Its relentless bombardment and growing ground offensive has left more than 11,500 people dead, many of them women and children, according to the Gaza health ministry.

Thursday's statement, signed by nearly a dozen heads of UN agencies -- including humanitarian affairs, human rights, migration, refugees, children, and health -- highlighted that the "intense hostilities and large-scale destruction of civilian infrastructure" had already displaced nearly 1.6 million people in Gaza.

The UN has criticised Israel's calls for civilians in Gaza to evacuate to so-called safe zones in the south, warning that nowhere in the overcrowded territory is currently safe.

"None of the humanitarian organisations we represent has been involved in preparing for the arrival of displaced people in any prospective 'safe zone' -- or 'humanitarian zone' -- in Gaza," Thursday's statement said.

The organisations were all committed to providing assistance and protection, they said, but warned that "without the right conditions, concentrating civilians in such zones in the context of active hostilities can raise the risk of attack and additional harm."

"No 'safe zone' is truly safe when it is declared unilaterally or enforced by the presence of armed forces."

The signatories said they could not take part in establishing zones "unless fundamental conditions are in place to ensure safety and other essential needs are met".

A mechanism would also need to be in place to supervise the implementation, they said.

In addition, they stressed that "any discussions around 'safe zones' must not detract from the parties' obligation to take constant care to spare civilians -- wherever they are -- and meet their essential needs".

"The civilian population in Gaza must have access to the essentials for survival, including food, water, shelter, hygiene, health, assistance, and safety," the statement said.

"Humanitarian organisations must have access to fuel in sufficient quantities to deliver aid and provide basic services," it added.

*This story was edited by Ahram Online