The situation in Gaza is "about as bad as it can get," Sari Bashi, program director of Human Rights Watch, told ABC News Live Sunday.

"People are being forced to drink brackish, untreated water because the price of bottled water has gone up beyond what most people can afford," Bashi said. "Food is short, there is no electricity. Even emergency generators that hospitals have are running low on fuel, and supplies are dwindling."

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While on a bipartisan congressional delegation trip to Israel, Senate Majority Leader Chuck Schumer and other members of the delegation were rushed into a bomb shelter in Tel Aviv to wait out a rocket attack, according to a social media post from Schumer.

"It shows you what Israelis have to go through," Schumer said in his post on X, which included a photo showing him and Republican Sen. Mitt Romney, a Romney staff and several others huddled in the small bomb shelter.

The U.S. Embassy in Jerusalem on Sunday morning urged U.S. citizens trapped in the Gaza Strip to move to southern Gaza along the main highway, the Salah al-Din Street.

U.S. citizens who can do so safely were advised by the embassy to flee to the south of the Gaza Valley toward the city of Khan Younis as U.S. officials work on potential options for getting them out of Gaza.

As of Sunday morning, the only exit to the south of Gaza, the Rafah crossing at the Egyptian border, remained closed to all seeking to flee Gaza, officials said.

But Tala Herzallah, a Palestinian woman trapped in Gaza, told ABC News Sunday that Israel Defense Forces are continuing to bomb both Khan Younis and Rafah, saying homes were hit in the areas Saturday night.

"They’re telling us to move there, and then they are bombing the houses there without warning people," Herzallah said.

The military conflict between Israel and Hamas is ongoing, making identifying departure options for U.S. citizens complex, the U.S. Embassy said.

Embassy officials encouraged U.S. citizens to complete a crisis intake form to provide contact information "should we need to reach out to you on short notice."

-ABC News' Clark Bentson

A Gaza resident said Hamas fighters were attempting to slow the evacuation of northern Gaza, using measures that included stealing car keys, Israel's military said.

"They are preventing people from leaving," the Gaza resident told an Israeli military officer in a phone call, the audio of which was somewhat garbled and which was recorded, translated and released by the Israeli Defense Forces. The audio appeared to be a clip of a longer conversation.

The man, whom IDF identified only as a resident of Jabalya, in northern Gaza, said Hamas has been "taking the ID card and the car keys" of some locals. Israel has ordered some 1.1 million people in northern Gaza to evacuate immediately ahead of an expected ground invasion.

"You mean the movement? Hamas?" a person identified as an IDF officer said in the recording. "Who is stopping you? Hamas?"

"Yes, yes," the Gaza resident replied.

The World Health Organization is begging Israel to immediately reverse its evacuation order for the over 1 million living north of Wadi Gaza, saying "a mass evacuation would be disastrous for patients, health workers and other civilians left behind or caught in the mass movement."

"With ongoing airstrikes and closed borders, civilians have no safe place to go," the WHO said in a statement. "Almost half of the population of Gaza is under 18 years of age. With dwindling supplies of safe food, clean water, health services, and without adequate shelter, children and adults, including the elderly, will all be at heightened risk of disease."

"Of the thousands of patients with injuries and other conditions receiving care in hospitals, there are hundreds that are severely wounded and over 100 who require critical care," the WHO said. "Many thousands more, also with wounds or other health needs, cannot access any kind of care. The compressed timeframe, complex transport logistics, damaged roads, and, above all, lack of supportive care during transport all add to the difficulty of moving them."