Defense officials have ordered a “complete siege” of Gaza after militants broke through its border wall into southern Israel on Saturday and killed at least 1,200 people, many of them civilians — the deadliest day in Israeli history.

“No power, no food, no gas,” Defense Minister Yoav Gallant said. “We are fighting human animals, and we act accordingly.” Hamas, the militant group that controls Gaza, orchestrated the ambush over the weekend, when militants took more than 100 hostages to Gaza.

Israel has bombarded the densely populated enclave in retaliation, killing at least 1,100 people. On Tuesday, the Israel Defense Forces announced that the border with Gaza was secured and that it was preparing for an offensive.

The Gaza Strip, a 140-square-mile stretch of land with more than 2 million people, depends on Israel for most of its electricity and other basic services. Cutting off gas and power from the territory could leave many residents not only without power, but without clean drinking water, proper sanitation and health care. On Wednesday, Palestinian authorities said Gaza’s only power plant has run out of fuel and stopped operating.

United Nations Secretary General António Guterres condemned the siege Monday. “The humanitarian situation in Gaza was extremely dire before these hostilities; now it will only deteriorate exponentially,” he said in a statement.

Israel provides Gaza with power in two ways. Gaza’s sole power plant is operated by diesel fuel, which enters Gaza from the Karem Shalom crossing. There are also 10 direct power lines from Israel to Gaza.

On Saturday, Gaza’s power authority reported an electricity shortage of close to 80 percent.

“Within a matter of days Gaza will be in a blackout,” said Miriam Marmur, public advocacy director at Gisha, an Israeli nonprofit organization.

The Israeli siege could also “result in a severe shortage of already scarce drinkable water,” the United Nations said. The country has several major desalination plants, which transform water from the Mediterranean Sea into drinkable water, but they need power to operate.

Sanitation facilities, too, could be crippled by the outages, increasing concerns about disease. Past fuel shortages in Gaza have caused sewage from the strip to contaminate agricultural land and spill into Israel.

Gaza “was on the edge of a humanitarian crisis for a very long time,” said Arnaud Quemin, the Middle East regional director of Mercy Corps. “So this additional shock is going to turn this into a full-blown crisis very quickly.”

Even before this escalation, residents in Gaza spent almost half the day, on average, without power.

Israel withdrew its civilian population and military forces from the Gaza Strip in 2005, but has continued to control the flow of goods and people. When Hamas assumed political power of the territory in 2007, Israel imposed an air and sea blockade, which is also enforced by Egypt to the south.

Over the course of the 16-year blockade, Israel has imposed shutdowns during key moments in the conflict, including Operation Cast Lead, which began in December 2008, and Operation Protective Edge in 2014. Those military campaigns killed more than 3,500 Palestinians in total.

“Israel used its control over the access to goods and infrastructure needs as a way of creating a carrot-and-stick approach to the governing body in Gaza, Hamas,” said Ghaith al-Omari, a senior fellow at the Washington Institute for Near East Policy. “If things are calm, things are opened up. If things get tense, Israel shuts the spigot.”

The siege could also affect hospitals, which can run on generators but need fuel to power them. Several health care facilities were already hit by Israeli strikes.

“We are very concerned to see that medical facilities have not been spared,” Léo Cans, Doctors Without Borders’ head of mission in the Palestinian territories, said in a statement. “[An] airstrike destroyed an ambulance carrying the wounded, right in front of the hospital where we work.”

Israel has justified its overwhelming use of force by saying it must destroy Hamas’s military capabilities to prevent any future assaults on the country.

“It will never happen again,” Prime Minister Benjamin Netanyahu said in a national address Saturday.

“We’re talking about Israeli policies,” al-Omari said, “but ultimately, Hamas knew exactly what was going to come.”