GAZA CITY, Palestine 

More than 400,000 Palestinians have been internally displaced in the Gaza Strip amid a massive Israeli air campaign, the UN humanitarian office said Saturday.

“Most people have no access to clean drinking water after supply through the water network and the operation of water desalination/purification plants came to a halt,” the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.

“As a last resort, people are consuming brackish water from agricultural wells, triggering serious concerns about the spread of waterborne diseases,” it added.

The Israeli military warned 1.1 million residents in northern Gaza to evacuate Friday “within 24 hours” and move south.

The OCHA said tens of thousands of residents are estimated to have fled south following the order.

The UN warned it would be impossible for Palestinians in Gaza to obey the order to leave the north without “devastating humanitarian consequences.”

Israeli forces launched a sustained and forceful military campaign against the Gaza Strip in response to a military offensive by the Palestinian group Hamas in Israeli territories.

The conflict began last Saturday when Hamas initiated Operation Al-Aqsa Flood -- a multi-pronged surprise attack including a barrage of rocket launches and infiltrations into Israel via land, sea and air.

Hamas said the operation was in retaliation for the storming of the Al-Aqsa Mosque in occupied East Jerusalem and Israeli settlers’ growing violence against Palestinians.

The Israeli military then launched Operation Swords of Iron against Hamas targets within the Gaza Strip.

That response has extended into cutting water and electricity supplies to Gaza, further worsening the living conditions in an area that has reeled under a crippling siege since 2007.

More than 3,300 people have been killed since the outbreak of the conflict, including 1,900 Palestinians and 1,400 Israelis.