Israel’s military on Friday called for all civilians of Gaza City, more than 1 million people, to relocate south within 24 hours, as it amassed tanks near the Gaza Strip ahead of an expected ground invasion.

The Israeli military has announced it would continue to operate significantly in Gaza, warning its 2.3 million residents to evacuate to the south of the enclave as it launches a barrage of bombs on the city.

The army said the residents will only be allowed to return to their homes when Israel makes another announcement permitting them to do so.

The bombardment is the most intense Israel has ever directed at Gaza, which is only 40km (25 miles) long and home to about one million children living in cramped cities and refugee camps with few designated shelters and nowhere else to run.

Explosions from air strikes echo every few minutes – killing more than 1,500 people in Gaza so far including hundreds of children, local health authorities say.

“Now is a time for war,” Defense Minister Yoav Gallant said on Thursday as Israeli warplanes continued pounding Gaza, destroying entire neighborhoods and killing hundreds of civilians.

The Israeli military said it would operate “significantly” in Gaza City in the coming days and civilians would only be able to return when another announcement was made.

The United Nations said it considered it impossible for such a movement to take place “without devastating humanitarian consequences.”

“The United Nations strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation,” UN spokesman Stephane Dujarric said in a statement.