Haditha Dam, outside of the Iraqi city by the same name, provides a critical power source for much of Anbar province.

BEIRUT — Militants from the Islamic State now control or threaten key facilities on the Euphrates and Tigris rivers, generating fears that the Al-Qaeda splinter group could turn off the taps to the Shiite south of Iraq, sparking a massive humanitarian crisis.

Last month's offensive across Iraq saw it overrun cities and battle for oil refineries as the national army melted away, but it has also been waging a war for water, trying to wrest control over rivers, dams and desalination plants in a bid to solidify its territorial gains.

Control of water is seen as key to the viability of the fledgling caliphate declared by Abu Bakr al-Baghdadi, leader of the group commonly known by many as ISIL. Without water, seasonal droughts cannot be managed, electricity cannot be generated, proper sanitation practices are near impossible and the local economy grinds to a virtual halt.

"When it comes to creating an Islamic state, it is not just about the control of geographic areas in Syria and Iraq. In order to form a viable state, one must control the state's most vital infrastructure, which in Iraq's case is water and oil," said Matthew Machowski, a Middle East security researcher in the British parliament and at Queen Mary University.

In Mosul, the first city ISIL captured, residents fled when the water and electricity were cut off but returned a few days later when the jihadist group was able to switch supplies back on, in a bid to engender support among the local population.

Iraq's 32 million people are entirely dependent on water flowing down from two great rivers in Turkey, the Euphrates and the Tigris. Where those waterways enter Iraq in the north, ISIL holds key dams and surrounding areas, leaving Shiite-majority southern Iraq vulnerable to the use of water as a strategic weapon.

In April, ISIL seized control of the Fallujah dam and its fighters released a wall of water from behind the barrage, destroying cropland 160 kilometers downstream and leaving millions of people without water in the predominately Shiite cities of Karbala, Najaf and Babil, while flooding areas as far away as Abu Ghraib.

"The intent behind the water release was to use water aggressively as a tool of destruction, targeting populations who live father south," said Russell Sticklor, co-author of Water Challenges and Cooperative Response in the Middle East and North Africa.

" ISIL is well aware of the strategic importance of controlling water access ... Control of this water infrastructure allows ISIL to control the faucet, and decide how much -- or how little -- water is released downstream. This is of great strategic importance because southern Iraq, the Shiite heartlands, needs water from the Tigris and the Euphrates to survive," he added. "They are in a very vulnerable position,"

U.N. Secretary-General Ban Ki-moon last week described the use of water as a weapon in Fallujah a "dangerous trend."

Like Fallujah, fears abound that militants could employ a similar tactic at the Samarra barrage, overtaken recently in clashes around the contested holy city, the site of bloody sectarian violence in 2007. The barrage is designed to control the flow of water from the country's biggest lake, Tharthar, and generate hydroelectricity.

With these key dams under its belt, ISIL appears to have turned its focus to Haditha, at the heart of Iraq's water infrastructure and responsible for 30 percent of the country's electricity, particularly to Baghdad. Positioned on a main artery to the capital, its capture would represent "a huge symbolic and practical victory," said Sticklor. Government troops are actively defending Haditha, alarmed that an ISIL victory there would pave a virtually clear the road to the capital.

Highlighting the importance of Haditha, the dam became one of the first areas secured by U.S. special forces in the 2003 invasion, amid concerns in Washington that Saddam Hussein would flood surrounding areas, explained Nouar Shamout, researcher at Chatham House.

Haditha, along with Mosul dam, provides more than 75 percent of Iraq's electrical power. Although the city of Mosul is firmly in ISIL hands, the dam further north is controlled by Kurdish forces, the peshmerga.

Last week, the peshmerga overran several oil fields around Kirkuk, expelling government staff, a move that could add 250,000 barrels a day of the Kurdish Regional Government's oil production.

Control of Mosul dam and the oil fields bring the Kurdish dream of establishing an independent state closer to reality, said Machowski.

" Mosul dam is absolutely essential to water security for Kurdistan. Securing the dam and oil installations puts the Kurds in a situation where independence becomes a fait accompli," he said.

With Kurds and ISIL both siphoning off key territory and resources, "the question of water is as serious as it gets, it really is an existential issue for Iraq," said Michael Stephens, Deputy Director of Royal United Services Institute Qatar, a British security think-tank.

Although ISIL has swept through territory holding vital water installations with apparent ease, questions have emerged over whether its fighters' capacity for brutal acts of violence can be matched by the technical know-how required to properly manage infrastructure and keep state facilities running smoothly.

Across the border in Syria, water in the ISIL-held Lake Assad is running dangerously low. The 85 kilometer long lake is Syria's largest and until recently it held more than 14.2 billion cubic meters of water. According to the anti-regime activist group the Raqqa Media Center, water levels have dropped by 1.6 billion cubic meters in the past few weeks, forcing two-thirds of the lake's electricity-generating turbines to stop working.

The dramatic, unprecedented drop in water levels has left nearly three million people in Aleppo and over a million people in Raqqa without potable water, Shamout said.

"Studying this case shows that ISIL does not have a strong water management policy ... Running a local water supply network is totally different from running a huge and interconnected water structures such as dams."

Apparently aware it lacks specialized knowledge in engineering and hydrology, the group negotiated to retain staff and keep the dam running at the Tabqa dam outside Raqqa.

The move appears shrewd. ISIL can retain the support of locals while still exerting influence over water distribution and receiving a steady stream of revenue to fund its tentative state -- it echoes the group's earlier tactic of allowing the sale of oil and oil derivatives back to the Damascus government from fields it had captured.

" ISIL know water, and water access, can be wielded as a powerful psychological weapon," Sticklor said. "The mere idea that your water could be cut off is deeply unsettling ... In the end no one can be certain of what the region's future holds. But if Iraq is descending into a civil war, water is guaranteed to emerge as a major component of this conflict."