Paula Gaviria Betancur, the United Nations' special rapporteur on the human rights of internally displaced persons, said it was inconceivable that Gazans could move across a war zone without devastating consequences.

Thousands of Palestinians fled to southern Gaza in search of refuge on Friday after Israel warned them to evacuate before an expected ground offensive against Hamas in retaliation for the deadliest attack in Israel's history.

"Forcible population transfers constitute a crime against humanity, and collective punishment is prohibited under international humanitarian law," Gaviria Betancur said in a statement.

"We are horrified at the prospect of an additional one million Palestinians joining the over 423,000 people already forcibly driven from their homes by the violence over the past week," she said.

"It is inconceivable that more than half of Gaza's population could traverse an active war zone, without devastating humanitarian consequences, particularly while deprived of essential supplies and basic services."

UN special rapporteurs are unpaid, independent figures mandated by the Human Rights Council. They do not speak for the UN but report their findings to it.

Gaviria Betancur urged Israel to rescind the evacuation order immediately, saying those currently displaced had nowhere to go.

The Colombian human rights lawyer said tripling the displaced population overnight would "permanently alter the civilian population of Gaza".