Israel's military admitted Friday it would take time for Palestinians to follow its orders to evacuate north Gaza, but did not confirm a UN report that it had set a 24-hour deadline.

"We are trying to provide the time and we are doing a lot of effort, and we understand it won't take 24 hours," army spokesman Daniel Hagari told journalists.

"I will not say (if) it's an accurate time," he added, when asked to confirm the army had informed UN officials Gazans had 24 hours to flee.

The United Nations warned the displacement of 1.1 million residents from northern Gaza could lead to "devastating humanitarian consequences", while the World Health Organization said it is impossible to evacuate vulnerable patients.

The evacuation order comes as Israel pounds the blockaded Gaza Strip with strikes and Palestinian militants fire rockets towards Israel.

The war erupted on Saturday when Hamas militants crossed from Gaza and launched attacks on border communities, killing around 1,200 people.

The retaliatory strikes on Gaza have killed more than 1,530 people, according to the health ministry in the Palestinian territory.