The Islamic State militants who have rampaged across northern Iraq are increasingly using water as a weapon, cutting off supplies to villages resisting their rule and pressing to expand their control over the country’s water infrastructure.

The threat from the jihadists is so critical that U.S. forces are bombing the militants close to both the Mosul and Haditha dams — Iraq’s largest — on a near-daily basis. But the radical Islamists continue to menace both facilities.

The Sunni militants want to seize the dams to bolster their claim they are building an actual state; the dams are key to irrigating the country’s vast wheat fields and providing Iraqis with electricity. More ominously, the Islamic State has used its control over water facilities — including as many as four dams along the Tigris and Euphrates rivers — to displace communities or deprive them of crucial water supplies.

The Islamic State “understands how powerful water is as a tool, and they are not afraid to use it,” said Michael Stephens, a Middle East expert and deputy director of the Royal United Services Institute, a London-based security studies think tank.

“A lot of effort has been expended to control resources in Iraq in a way not seen in other conflicts,” he added.

Water has long played a role in armed struggle, from the Allied bombing of German dams during World War II to Saddam Hussein’s draining of Iraq’s southern marshes in the 1990s to punish residents for an anti-government rebellion.

But the idea of a radical, non-state group gaining authority over critical water infrastructure has raised particular worry. The White House was so alarmed in August when Islamic State fighters briefly seized the Mosul Dam — located on the Tigris River that runs through Baghdad — that it backed a major operation by Iraqi and Kurdish forces to wrest it back.

“If that dam was breached, it could have proven catastrophic, with floods that would have threatened the lives of thousands of civilians and endangered our embassy compound in Baghdad,” President Obama said on Aug. 18, on the day Iraqi forces retook the structure.

Having nurtured the world’s first civilizations along the Fertile Crescent — the ancient strip of food-bearing land that arced across the Middle East — Iraq’s Tigris and Euphrates rivers remain the lifeblood of Iraq’s agricultural life. They also generate its electricity and provide water that is piped in to households.

But water levels in Iraq have fallen in recent years, due to decreased rainfall, heavy water use and other factors, the United Nations says. According to the world body, the flow of the Euphrates is expected to decline by more than 50 percent by 2025. By then, Iraq could already be suffering from a shortage of 33 billion cubic meters of water per year, U.N. officials say.

“The country does not have enough [water], and shortages have been huge economic — and thus political — problems for several years now,” said Kenneth Pollack, an expert on Middle Eastern military affairs at the Brookings Institution. Any attempts by the Islamic State to cut flows “would be enormously damaging,” he said.

The Sunni extremists of the Islamic State say Shiite Muslims are apostates. In Iraq, the militants accuse the Shiite population of backing a sectarian government that has oppressed Sunnis.

In April, Islamic State jihadists controlling the Fallujah dam in western Anbar province closed its gates, in a move that some Iraqi officials say was meant to slow the flow of water to the Shiite-dominated provinces in the south.

But the subsequent buildup of water at the Fallujah dam ended up flooding an irrigation channel in a Sunni area nearby, sending a wave of water into homes, schools and farmland. The deluge — which also swept away livestock and sent residents scrambling for makeshift rafts — ended up affecting as many as 40,000 people, aid workers said.

Last month, the Islamic State used its control of the Sudur mini-dam north of Baghdad to cut off water to Balad Ruz, a predominantly Shiite area of Diyala province. According to the town’s mayor, who spoke to an Iraqi news agency in September, the militants lined the roads to the dam with improvised explosive devices, and the government was forced to hire trucks to bring potable water to the residents.

Recently, a local official in Diyala province said Islamic State militants flooded nine villages in the Shirwain area by diverting water from nearby rivers, in order to prevent the advance of Iraqi security forces.

“We are in a conflict with the Islamic State over water in Iraq. They want to control it at any price,” said Abdul Majid Satar, the minister of agriculture and water resources for the Kurdistan Regional Government, which administers a semi-autonomous area in northern Iraq.

“They can threaten many parts of the country if they control the water,” Satar said.

The Islamic State militants captured Mosul, Iraq’s second-largest city, in June, and two months later expanded their offensive even further into northern Iraqi territory.

Many of the areas occupied in that August offensive have since been retaken by Kurdish pesh merga forces with the help of U.S. airstrikes. But when the jihadists departed, they used their control of the water and power networks in Mosul to shut off water and electricity to those areas, which are connected to the same grids.

“We came back to our villages and when we saw there was no power or water, we left again,” said Mazoot Shaqer Mohammad, a Kurdish farmer from the northern Iraqi district of Gwer, one of the areas that Kurdish fighters took back.

“Even when they withdraw, they are still in power,” Mohammad said of the militants. “They are not occupying land. But now they are controlling the return of people to these villages.”

In a telephone interview, a longtime employee of the Mosul water directorate, now under the control of Islamic State, was guarded in talking about the shutoff of water to certain villages.

“All I know is that we always supplied these villages with water, and now we can’t,” the employee, who gave his name only as Salah. “But I do believe the armed group [the Islamic State] is using water as a weapon,” he said.

In another small village near Gwer, the Islamic State took a different approach.

In the wheat-farming hamlet of Talkhaneim, the jihadists retreated but shut off the power that was used to draw water from the two local wells. Then the militants contacted a local official to say they would turn it back on if there was payment, according to a Kurdish resident and farmer, Ibrahim Ismail Rasool.

“They asked for 4 million dinars ($3,500) to turn the electricity back on. They are acting like a government, collecting bills,” said Rasool, his face bronzed from years toiling in the sun, in a recent interview.

Without water, none of the residents could return home, nor could they maintain their livestock. Rasool said he and the other villagers asked Kurdish officials if they could pay the Islamic State to get their power and water back.

“The government said no. That they don’t want to deal with Daash,” he said, using the Arabic acronym for the Islamic State. “But it’s fair that if they supply me with electricity, I should pay them.”