The head of the United Nations agency for Palestinian refugees has accused Israel of laying the groundwork for the mass expulsion of Gazans across the territory's border into Egypt.

In an opinion piece published on Saturday in the Los Angeles Times, UNRWA chief Philippe Lazzarini pointed to the worsening humanitarian crisis in Gaza and the increasing concentration near the border of displaced civilians who fled the fighting, first in the north and then further south.

"The United Nations and several member states, including the US, have firmly rejected forcibly displacing Gazans out of the Gaza Strip," Lazzarini said.

"But the developments we are witnessing point to attempts to move Palestinians into Egypt, regardless of whether they stay there or are resettled elsewhere."

The widespread destruction in the Palestinian territory's north and the resulting displacements were "the first stage of such a scenario", he added, while forcing civilians from the southern city of Khan Yunis closer to the border was the next.

"If this path continues, leading to what many are already calling a second Nakba, Gaza will not be a land for Palestinians anymore," Lazzarini said, using the Arabic term for the exodus or forced displacement of 760,000 Palestinians in 1948.

A spokesperson for the Israeli defence ministry office responsible for Palestinian civilian affairs did not immediately respond to a request for comment on Lazzarini's accusation.

When asked about the possibility of evacuating people into Egypt last week, a government spokesman said Israel was "focusing on getting civilians out of harm's way inside the Gaza Strip".

Nearly 85% of Gaza's 2.3 million people have been displaced within the besieged territory, where U.N. agencies say there is no safe place to flee.