Israel expanded the zone in May, to include parts of Khan Younis and Deir al-Balah, after launching its ground offensive in Rafah, where more than a million people were sheltering.
But it has been repeatedly reduced in size since then, as Israel has targeted areas where they say Hamas is operating and launching attacks while embedded among displaced civilians.
An estimated 1.8 million people are now sheltering in al-Mawasi, which currently spans about 50 sq km (20 sq miles) and lacks critical infrastructure and basic services.
Overcrowding has become a major concern in UN emergency shelters in central and southern Gaza, with some far exceeding their capacity. Other families are living in tents or makeshift shelters in compounds or on open areas of land and beaches.