With a potential ground offensive maybe just hours away, Israel has told more than a million people in northern Gaza to flee south to escape the fighting -- a move the United Nations said could have "devastating humanitarian consequences."

The Israeli Defense Forces early Friday called for civilians to evacuate to the south of Wadi Gaza, citing plans to "operate significantly" in Gaza City after Hamas unleashed unprecedented terror attacks on Israel this past weekend.

The IDF said it was telling residents to leave "for your own safety and the safety of your families." At the same time, Hamas, which has ruled the Gaza Strip for more than a decade, told civilians to stay put, prompting Israel and the U.S. to accuse the militant group of wanting to use civilians as human shields.

Even if civilians are able to escape south of the Wadi Gaza river, there are currently no viable options for them to leave Gaza entirely as border crossings at Rafah to the south and Erez in the north remain closed.

White House spokesperson John Kirby called Israel's move a "tall order" when asked whether it was even possible.

"They're trying to move civilians out of harm's way and giving them fair warning," Kirby said on CNN. "Now it's a tall order. It's a million people, and it's a very urban, dense environment, already a combat zone. I don't think anybody's underestimating the challenge here of affecting that evacuation."

The Biden administration previously said it was pressing for safe passage for civilians to escape Gaza through Rafah -- the sole border crossing between Egypt and Gaza.

But as of Friday, the benchmarks for safe passage being negotiated between the U.S., Israel and Egypt appeared to shrink.

According to a senior State Department official, after a week of effort, the U.S. was still working with Egyptian and Israeli counterparts to create a corridor for Americans and other foreign nationals to leave -- but not Palestinians.

In a shift, the U.S. said it was also working with humanitarian organizations like the Red Cross and the U.N. to establish "safe zones" inside Gaza for civilians, the official said.

Secretary of State Antony Blinken, during a news conference in Qatar, said safe areas are a "priority" but that efforts to get humanitarian assistance into Gaza are being complicated by Hamas. Blinken said the group is using "innocent civilians as human shields and is reportedly blocking roads to prevent Palestinians from moving to southern Gaza out of harm's way."

"Civilians of course should not be the target of military operations," Blinken said. "They are not the target of Israeli operations. They are very deliberately the target of Hamas' actions."

Israel's call for an evacuation was met with criticism from some humanitarian organizations as well as the United Nations, which said it was informed just after midnight local time that the entire population of northern Gaza should be evacuated.

"The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences," Stéphane Dujarric, the spokesman for U.N. Secretary-General António Guterres, said in a statement. Dujarric said the U.N. said it was strongly appealing for the request to be rescinded, to avoid a "calamitous situation."

The Palestinian Red Crescent Society, which is providing medical aid in Gaza, called the IDF call to evacuate "shocking and beyond belief" and said they don't have the means to evacuate the wounded, the elderly or the disabled. Doctors Without Borders called it "outrageous."

The conflict, now in its seventh day, has left more than 3,000 people dead on both sides. In Israel, at least 1,300 people have been killed and another 3,227 injured. In Gaza, at least 1,799 people have been killed, including hundreds of women and children, and more than 7,000 people have been injured.

Blinken said the U.S. was "very actively engaged with U.N. relief agencies, the ICRC and others who address the huge humanitarian needs of people in Gaza, to protect them from harm and make sure that they have the ability to get what they need."

President Joe Biden has said he emphasized in a call with Israel Prime Minister Benjamin Netanyahu that Israel has the right to defend itself but has to "operate by the rules of war."

James Jeffrey, the chair of the Middle East Program at the Wilson Center and a veteran diplomat who served as ambassador to Iraq, said the evacuation order may be Israel's "initial response" to that call but issues remain as to its feasibility.

"This obviously needs to be fleshed out," Jeffrey, who has experience with evacuations in war-torn areas, told ABC News. "At the minimum there would need to be an overall ceasefire and/or designated safe routes."

"It's not directly, under the laws of war, the Israeli’s problem what happens to these people when they move to the south but it is an important political and diplomatic issue for both Israel and the U.S.," he added.

ABC News' Shannon Crawford contributed to this report.