Two million Gazans are now confined to 15 square miles

Israel has in recent weeks widened its offensive against Hamas in Gaza to areas its military previously marked as safe zones, but where it now says militants are hiding, hemming Palestinians into smaller and smaller portions of the strip.

So far this month, the Israeli military has issued at least nine evacuation orders covering areas it has designated as humanitarian zones, directives that the United Nations estimates have affected 213,000 people. It means Gaza’s 2.2 million people are now mainly confined to an area of roughly 15 square miles—smaller than the footprint of Manhattan.

Put another way, at the start of the year, evacuation orders pushed Palestinians fleeing the war to shelter in areas totaling around 33% of the strip, according to the U.N.; they are now reduced to just 11% of Gaza, an enclave roughly the size of Philadelphia.

The shrinking space for Palestinians to seek refuge is causing heightening fears over the outbreak of disease and worsening living conditions in the small pockets available to shelter. “This means there is going to be more disease, more pressure on whatever facilities exist," said Bushra Khalidi, a policy lead at Oxfam, a charity working in the Gaza Strip.

Israel says it hopes to find and kill Hamas militants in areas it had designated to be humanitarian zones after sweeping through much of the rest of the strip. But the smaller space risks worsening an already dire humanitarian crisis and heaping international pressure on Israel to end the war.

U.S. and Arab mediators are pushing Israel and Hamas to agree to a cease-fire that would alleviate the situation. But the sides remain far apart.

For months, Israel has warned that Hamas militants are using an Israeli designated-humanitarian zone known as Al-Mawasi to launch rocket attacks and regroup. Israeli airstrikes have caused civilian casualties in the area and led some Palestinians to conclude that no part of the strip is safe.

Hamas “continues to deliberately embed its military assets used to carry out attacks against Israel next to humanitarian infrastructure and the civilian population," the Israeli military said ahead of a recent evacuation order. The military didn’t respond to a request for comment about the humanitarian impact of reducing the space designated for civilians.

To avoid further civilian deaths, Israel is asking people to squeeze into a smaller stretch of Al-Mawasi that aid groups say is already packed with Palestinians living in tents. There is little medical infrastructure, and food and clean drinking water are scarce, these groups say.

Dua’a Abu Daqqa, 35, who is disabled, recently moved to Al-Mawasi after receiving an evacuation order, but returned after just a week to a tent near the southern Gazan city of Khan Younis. Being disabled has made the multiple moves around the strip particularly hard, she said. “My wheelchair was stolen, we have to find a car every time we are asked to evacuate, which isn’t available," Abu Daqqa said. “I sometimes have to be carried to be able to escape death."

A new concern about the concentration of people is poliovirus. The Israeli military earlier this year said it had found remnants of poliovirus in sewage in Gaza, and the enclave’s health ministry last week said it had found traces of the virus, which can lead to paralysis, in a 10-month-old child.

U.N. Secretary-General António Guterres has said that hundreds of thousands of children in Gaza are at risk of contracting polio, and the Israeli military this week held a meeting with aid agencies about how to vaccinate against the virus, which has been largely eradicated in most of the world.

Untreated sewage, shortages of clean water, insufficient food, compromised medical facilities and a lack of personal hygiene supplies are exacerbating fears over an outbreak. The recent Israeli evacuation orders have affected makeshift clinics, schools and water-and-sanitation facilities, the U.N.’s humanitarian coordination office said Monday.

“This reduction in space, combined with overcrowding, heightened insecurity, inadequate and overstretched infrastructure, ongoing hostilities, and limited services is exacerbating the dire humanitarian situation," the U.N. said.

Israel has said it is doing all it can to allow aid into Gaza but says it is hindered by Hamas, which it says launches rockets from sites near aid-distribution warehouses.

More than 40,000 Palestinians, most of them civilians, have been killed since Israel began aerial bombardment and ground operations in Gaza last year, according to Palestinian officials, who don’t specify how many were combatants.

Israel invaded after the Oct. 7 Hamas-led attack that killed 1,200 people, mostly civilians, and took more than 240 hostages, according to Israel. Israel’s military campaign has largely moved from north to south over the 10-month course of the war, forcing Palestinians into the Al-Mawasi area in the west of the strip.

In recent months, the Israeli military has focused on the southern Gaza city of Rafah, but it has also said it has faced Hamas attacks from inside the so-called humanitarian zone. The Israeli military this week also said it recovered the bodies of six hostages from a tunnel beneath Khan Younis in an area that it had previously designated a humanitarian area.

Hamas has been retreating to so-called safe zones to rebuild its military capabilities and launch rocket attacks, according to Amir Avivi, a former deputy division commander of the Israeli military in Gaza.

“The whole Hamas strategy is to use its citizens as human shields and to hide in civilian infrastructure," Avivi said. By issuing evacuation orders, Israel “wants to minimize collateral damage," he added.

Hamas said on Wednesday that Israel was “deliberately suffocating" Palestinians by forcing them into Al-Mawasi.

Fighting has continued between Israel and Hamas in Gaza while the U.S. and Arab mediators have pushed the two sides to agree to a cease-fire deal.

The Israeli-designated humanitarian zone and the surrounding area have remained dangerous.

In July, Israel dropped eight 2,000-pound bombs at a target on the eastern flank of that area in an attempt to kill Hamas’s military leader and others. Gaza health authorities said more than 90 people were killed and hundreds injured. Israel has said the strike killed the group’s military head, Mohammed Deif, but Hamas has denied that.

Some 289 aid workers also have died in Gaza since Oct. 7, according to the U.N.

Forced to move at least two or three times already, some Palestinians are risking staying where they are, unable to find tents or shelter elsewhere and unconvinced that any part of Gaza is now safe.

“We know that we might be asked to leave soon," said Fatma Khalaf, a 37-year-old mother of three children in the central Gazan city of Deir al-Balah. “But we really don’t know where we will go."

Write to Rory Jones at Rory.Jones@wsj.com