Islamic State (IS) militants have closed some dam gates on the Euphrates River near Ramadi in western Iraq, reducing the vital flow of water to government-held areas while giving the militants greater freedom to attack government forces downstream.
The move on June 3 threatens the drinking water, irrigation water, and water-treatment plants for thousands of residents and troops in areas held by the Iraqi government.
And it also poses a threat to security forces fighting to recapture Ramadi.
If water levels drop significantly, said Anbar Province councilman Taha Abdul-Ghani, the insurgents could cross the Euphrates on foot and attack troops deployed along the river and stationed at nearby Habaniya military base.
The base has been used as a staging ground for Iraqi troops and allied Shi'ite militias in the fight to retake Ramadi.
It is not the first time the IS group has used water as a weapon. Earlier this year, it reduced the flow through a lock outside the town of Fallujah, also in Anbar Province, though it soon reopened the lock after criticism from residents.
Last summer, IS took control of the Mosul dam -- the largest in Iraq -- and threatened to flood Baghdad and other cities downstream. But Iraqi and Kurdish forces, backed by U.S. air strikes, later recaptured the facility.
Outside Ramadi, thousands of people in the government-held towns of Khalidiya and Habaniya are already suffering from shortages of drinking water because purification plants along the Euphrates have all but shut down due to low water levels caused by the summer heat.
The residents of the towns get only two hours a day of water through their pipes, Abdul-Ghani said.
"With the summer heat and lack of water, the lives of these people are in danger and some are thinking of leaving their homes," he said.
Abu Ahmad, who owns a vegetable farm near Khalidiya, said he could lose all his crops because of lack of irrigation water. Now, the water is lower than the level of his water-pumping machines.
"I used to irrigate my crops every three days. If the situation continues like this, my vegetables will die," said Abu Ahmad, using a nickname because of fears for his life.
The United Nations said on June 3 it was looking into reports that IS had reduced the flow of water through the Al-Warar dam.
"The use of water as a tool of war is to be condemned in no uncertain terms," said Stephane Dujarric, spokesman for the UN secretary-general. "These kinds of reports are disturbing, to say the least."
The move is also worrying for an array of troops fighting IS.
The Euphrates has acted as a barrier between the militants, who control its northern bank, and pro-government forces who are trying to advance toward Ramadi on the other side.
A spokesman for the governor of Anbar Province said security forces would now have to redeploy along the river to prevent the insurgents from infiltrating.
"Previously they had to monitor only the bridges and certain areas, but now all of the river will be crossable," Hikmat Suleiman said.
The government has found a temporary countermeasure. The partial closure of the Ramadi dam has forced more water into a tributary running south to Habbaniya lake, officials said.
Falih al-Issawi, a senior provincial security official, said the government had opened another dam to channel water from Habbaniya lake back into the Euphrates and prevent shortages in the southern provinces.
But he said this was only a temporary measure that would not be effective for more than three days.
"The government must act immediately otherwise dire consequences and an environmental catastrophe will be inevitable," he said.
With reporting by AP and Reuters