Israel-Gaza live updates: IDF says it's discovered one of Hamas' biggest tunnels

Israeli Defense Minister Yoav Gallant said Monday that Israel "will take any measures in order to destroy Hamas, but we have no intention to stay permanently in the Gaza Strip."

"We only take care of our security and the security of our citizens alongside the border with Gaza and elsewhere," Gallant said.

Gallant said "a new civil body will be established to try and look after the welfare of the residents."

"The key condition is that this body will not act with hostility towards the state of Israel," Gallant said. "All the rest, in my opinion, can be discussed. It certainly will not be Hamas, and also will not be Israel. We will maintain our freedom to act, to operate militarily against any threat."

Speaking directly to Hamas, Gallant said, "To the terrorists, to their commanders and to the battalion commanders: surrender. If you surrender, you can save your lives. If not, your fate is sealed."

Dr. Renana Eitan, a psychiatrist who treated people held by Hamas, said some of the now-released hostages experienced "cruelty that I haven't seen before."

"I've been a psychiatrist for over 20 years," she said. "We are [a] national center for sexual trauma and for PTSD for the refugees from Africa, so I thought I saw all the worst PTSD patients."

Some hostages were held "in inhumane sanitary conditions" and "subject to severe physical, sexual and mental abuse," Eitan said.

"I have never seen anything like that before," Eitan said.

"One of the patients, she was kept in total darkness for four days. This is inhumane. She became psychotic. She had hallucinations," Eitan said. "I've never seen such things in my life."

According to the Israel Defense Forces, 137 people are still being held hostage by Hamas.

A group of 18 protesters calling for a cease-fire in Gaza chained themselves to the White House fence on Monday.

The protesters were from Jewish Voice for Peace, a group that defines itself as "Jews organizing toward Palestinian liberation." They chanted, "Biden, Biden pick a side, cease-fire not genocide," and, "Cease-fire cannot wait, no Hanukkah to celebrate."

U.S. Park Police said its officers used bolt cutters to remove the chains from the fencing and cleared the group from the area after roughly 30 minutes.

The demonstration came hours before President Joe Biden holds a Hanukkah reception at the White House.

-ABC News’ Justin Ryan Gomez

The United Nations Relief and Works Agency for Palestine Refugees said 134 of its workers have been killed since the Israel-Hamas war began more than two months ago.

Although northern Gaza was the first region intensely targeted by Israeli forces, the UNRWA said half of its killed staffers died in central and southern Gaza.

There are 1.9 million people now displaced in Gaza, where conditions continue to deteriorate, the UNRWA said.

"Despite everything, our UNRWA colleagues keep going, keep doing everything they can to support their communities," the UNRWA said.