Approximately 1.3 million people are currently living in shelters in the war-torn Gaza Strip, according to head of the World Health Organization, who warned Wednesday that "a sustained ceasefire" between Israel and Hamas is "a matter of life or death for civilians" in Gaza.

WHO Director-General Tedros Adhanom Ghebreyesus said "overcrowding and lack of food, water, sanitation and basic hygiene, waste management and access to medication are resulting in a high number of cases of" various contagious diseases, including 111,000 cases of acute respiratory infections, 12,000 cases of scabies, 11,000 cases of lice, 75,000 cases of diarrhoea, 24,00 cases of skin rash, 2,500 cases of impetigo, 2,500 cases of chickenpox and 1,100 cases of jaundice.

"On top of all this: heightened risk of disease outbreaks," Tedros wrote in a post on X, the social media platform formerly known as Twitter.

"Given the living conditions and lack of health care, more people could die from disease than bombings," he added. "We need a sustained ceasefire. NOW. It's a matter of life or death for civilians."

Gaza, a 140-square-mile territory governed by the Palestinian militant group Hamas, is home to more than 2 million people.

-ABC News' Morgan Winsor

The Israeli Prime Minister's Office said Wednesday that 161 hostages are still being held in the Gaza Strip.

Of the 161 remaining hostages, 126 are male and 35 are female. Among them are 146 Israelis and 15 foreigners, including Americans. Ten of them are age 75 and older, four are 18 or 19, and four are under the age of 18, according to the Israeli Prime Minister's Office.

So far, 86 hostages -- 66 Israelis and 20 foreigners -- have been freed since last Friday in exchange for the release of dozens of Palestinians from Israeli prisons as part of a cease-fire agreement between Gaza's militant rulers, Hamas, and Israel. Another prisoner swap is expected to take place Wednesday.

-ABC News' Jordana Miller, Joe Simonetti and Morgan Winsor

The United States' Special Presidential Envoy for Hostage Affairs Roger Carstens, often referred to as the country's top hostage negotiator, will travel to Israel on Wednesday, according to a senior official in the U.S. Department of State.

The official said that Carstens "will support Secretary Blinken's visit to the region, meet with Israeli government counterparts and visit the families of Americans held hostage in Gaza."

U.S. Secretary of State Antony Blinken is expected to arrive in Israel on Wednesday night.

It will be Carstens' first known trip to Israel since Oct. 7, when Palestinian militant group Hamas carried out an unprecedented attack on southern Israel and took hundreds of people hostage back to the neighboring Gaza Strip. Carsten's deputy, Steven Gillen, visited Israel shortly after the war broke out and has spent a substantial amount time in the region during the weeks that followed, largely coordinating with Israeli officials on efforts to free the hostages.

Earlier this month, Carstens met with family members of some of the hostages in Washington, D.C.

-ABC News' Shannon Crawford and Morgan Winsor

White House National Security spokesperson John Kirby said there's no indication that Hamas is trying to keep Americans as hostages in order to use them as leverage.

"There's no indication that Hamas is trying to play some sort of game here in terms of the Americans," Kirby told reporters Tuesday.

He added that the first test case of getting hostages out involved two Americans, including a teenager from the Chicago area, and that 4-year-old American-Israeli hostage Abigail Idan was released over the weekend.

In the latest exchange on Tuesday, 12 hostages -- 10 Israelis and two foreign nationals from Thailand -- were released, the Israeli government said.

Kirby reiterated that the pool of American hostages is "pretty small, and the pool of Americans that qualify right now, women and children, is smaller still."

He also noted that there are issues locating the hostages, who may be held by various groups.

The U.S. believes there are eight or nine Americans still being held in Gaza, Kirby said on Monday.

-ABC News' Selina Wang and Cheyenne Haslett

Muhammad Ibrahim Asfour was among the Gaza residents who returned home during the cease-fire to find destruction where homes used to be.

"I was shocked, and when I saw the house I started crying," Asfour told ABC News. "There were 50 people in the building here -- all of my brothers and father were living here."

"The situation here is more difficult than you can imagine," Abdelkader Darma told ABC News. "When I came home I was shocked."

"People do not know where to go," Darma said. "I and six other people slept on two mattresses and two pillows, which means you do not know how to sleep. They sleep three against three, and you find his feet in front of your face."

“I hope the truce will be extended and a solution will be found because it is enough. People are sleeping in the street," Darma said. "How can you bear it?"

-ABC News' Zoe Magee and Sami Zayara