A Palestinian child looked on as smoke billowed from a bombardment in Rafah last month.

TEL AVIV—Israel’s defense minister outlined a plan this week that would go beyond anything Israel has previously done in Gaza, by screening hundreds of thousands of Palestinians before moving them into a secure area.

Now, the Israeli military’s legal branch and some of the country’s leading lawyers are raising concerns that the plan could expose Israel to accusations of forced displacement and internment of civilians, both illegal under international law.

Under the proposal unveiled by Defense Minister Israel Katz, the Israeli military would move Palestinians to a designated area around the southern Gazan city of Rafah, where they would reside and receive aid. The military would secure a perimeter and screen people upon entry to weed out alleged militants. Once inside, people wouldn’t be allowed to leave, he said.

The Israeli military’s legal branch has raised concerns over the idea, according to two military officials familiar with the matter, arguing it would be illegal to coerce civilians into the zone or prevent them from leaving it, coax them to leave Gaza, or withhold aid from other parts of Gaza once the plan is enacted. A group of military lawyers and other officers met with military chief of staff Eyal Zamir this week to explain the problems with the plan, the officials said.

In response to a request for comment on opposition to the plan, an Israeli military official said it would “act in accordance with the directive of the political echelon, and international law, in order to achieve the goals of the war.” Israel’s defense and justice ministries declined to comment on the plan’s legality.

Mourners at the funeral last month of a Palestinian killed in gunfire near an aid distribution center in Rafah.

Lawyers in Israel and abroad have warned the plan could be considered illegal under international law.

“The plan as expressed by Mr. Katz raises serious legal difficulties,” said Eran Shamir-Borer, who was formerly the Israeli military’s top international lawyer and is now at the Jerusalem-based think tank Israel Democracy Institute. He said he didn’t believe the plan would be implemented as Katz described it.

Three reserve soldiers, including a captain and a major, petitioned Israel’s supreme court to make the military clarify whether the military’s objectives for Gaza included the forcible removal of the population and whether it was illegal. Zamir, the military’s chief of staff, responded in a court filing this week by saying that the Israeli military “doesn’t enforce the transfer of the population inside or outside of the Gaza Strip.”

Israeli government officials have alluded to plans to move Gazans to the south, but Katz’s comments this week were the strongest signal yet they were serious about moving ahead. Israeli Prime Minister Benjamin Netanyahu said in May that Gazans would be evacuated to a “sterile zone” where they would receive aid.

Katz said the plan would initially involve the transfer of some 600,000 displaced Palestinians mostly from tent camps in a coastal area called Al Mawasi to a new settlement in the south. There are more than two million people living in Gaza.

Unlike at the camps in Al Mawasi, the Israeli military would control who could enter this new zone, secure its perimeter and prevent those inside from leaving. Operations inside the site would be run by “international entities,” Katz told reporters. Israel’s defense ministry declined to comment further.

The plan has been framed by officials as a way to get lifesaving aid to civilians without benefiting Hamas. Israel says Hamas steals aid and uses it to fund its war effort, which the group denies. Security analysts say the plan would also serve Israel’s military objectives by trying to separate civilians from militants, which they say would give troops operational freedom to keep fighting until they fully defeat Hamas.

An injured Palestinian girl and her siblings played inside their tent in the Al Mawasi settlement in the Gaza Strip.

Similar plans have circulated over the course of the war, often designed by retired army and intelligence officers and other experts who informally advise current officials. One such proposal was floated last year to create Hamas-free humanitarian “bubbles” across the strip, but like others it wasn’t adopted. These plans all sought to address the same problem, that it was difficult to fight Hamas as it hid amid civilians in dense urban centers.

Forcible displacement is a crime under the Geneva Conventions, to which Israel is party, and is permissible only in narrow circumstances such as temporary evacuation for civilian safety or military necessity. The bar for meeting that criteria is high, and the coercive environment of war-torn Gaza would undermine arguments that transfer is voluntary, according to Israeli and international legal experts.

Eyal Benvenisti, an Israeli lawyer who has defended Israel against allegations of genocide at the International Court of Justice in The Hague, said he “cannot see any lawful justification” for the plan that’s been proposed, emphasizing that it’s still under debate and faces opposition from military leadership.

Benvenisti co-signed a letter with 15 other Israeli experts in international law and the laws of war that criticized the plan as “blatantly illegal.” The letter, addressed Thursday to Israel’s defense minister and top general, called “on all relevant parties to publicly withdraw from the plan, renounce it and refrain from carrying it out.”

Since an earlier cease-fire collapsed in March, Israel has taken control of around 65% of the Gaza Strip, pushing civilians into an ever-smaller space. Most of the residents are now living in major population centers in northern and central Gaza. Israel has refrained from operating in these areas, security analysts say, over fears hostages are being held there, along with the danger to civilians and soldiers.

The site of the proposed zone is a patch of land in the south over which Israel has tightened its control. It includes Rafah, a city on the border with Egypt that’s now almost completely evacuated with entire neighborhoods demolished. To its south, Israel took control of a strategic belt along the border with Egypt called the Philadelphi corridor earlier in the war. To its north, Israel created another called the Morag corridor, giving the military control over everything in between.

Amir Avivi, the founder of the Israel Defense and Security Forum think tank and a former deputy commander, said his organization helped develop the plan more than a year ago. He said Israel’s framework for overhauling the aid system was always meant to be carried out in phases, providing for basic needs in the interim while preparations were laid for a large humanitarian zone that is free of militants, where aid can flow more freely.

This arrangement would also allow for a full siege of areas outside the humanitarian zone, he said, because only militants would be left there. “If it happens, and overall the society will move to one area…and Hamas will be completely detached from the society,” he said, “this is pretty much game over.”

Legal experts and human rights groups say it’s impractical to sort the whole population into either combatants or civilians. It also isn’t clear how Israeli soldiers would decide who can and can’t enter the humanitarian zone, or what would happen if civilians refuse to leave their homes.

Saher Al Sabbagh, 22, said he would be willing to leave his tent in Al Mawasi because he sees no hope in either staying where he is or resisting Israeli orders. “I’m ready to go because there is nothing left for us here—I mean, here or there, it’s the same,” he said. “They’ll force us out to wherever they want.”

Palestinians heading to receive humanitarian aid in May from a U.S. and Israeli-backed aid site in Rafah. Some crowds at aid sites met with gunfire after approaching Israeli military positions.

Others explicitly said they don’t want to relocate. Hazar Ahmad, 23, who returned to his home in Gaza City after having to pack up and flee three times since the war began, said he was unconvinced that moving again would provide him safety and he would rather stay at home. “There’s no such thing as a safe zone. The bombing happens everywhere,” he said.

The proposal also follows the troubled rollout of a U.S.-backed initiative, called the Gaza Humanitarian Fund, which was set up to distribute aid to Palestinians in the enclave. It came under criticism after massive crowds heading to aid sites were met with deadly violence, as Israeli soldiers opened fire toward crowds when they got too close to their positions.

Humanitarian workers are concerned the proposed zone would have similar problems on a larger scale, saying it’s dangerous to put Palestinian civilians in such proximity to Israeli soldiers and restrict them to an area under their watch.

Olga Cherevko, spokeswoman for the U.N. humanitarian office in Gaza, said the U.N. wouldn’t take part in any arrangement that goes against humanitarian principles.

“Any operation that channels desperate civilians into militarized zones is inherently unsafe,” she said.