Israel has bombarded the densely populated enclave in retaliation, killing at least 920 people. On Tuesday, the IDF announced the border with Gaza was secured and 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 country could leave many residents not only without power, but without clean drinking water, proper sanitation and health care.

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.

The Gaza Strip and its history, explained

Israel provides Gaza with power in two ways. Gaza’s only 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. The U.N. Office for the Coordination of Humanitarian Affairs estimated Monday that the power plant could soon run out of fuel.

“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 U.N. 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 of the conflict, including Operation Cast Lead that began in December 2008 and Operation Protective Edge in 2014. The 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,” Leo 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.”

Sources: U.N. Office for the Coordination of Humanitarian Affairs (OCHA), Gisha Legal Center for Freedom of Movement