The Islamic State group has reduced the amount of water flowing to government-held areas in Iraq's western Anbar province, an official said Thursday.

The reduced flow through an insurgent-held dam on the Euphrates River will threaten irrigation systems and water treatment plants in nearby areas controlled by troops and tribes opposed to the extremist group, provincial council member Taha Abdul-Ghani told The Associated Press.

Abdul-Ghani said there would be no immediate effect on Shiite areas in central and southern Iraq, saying water is being diverted to those areas from the Tigris River.

On Wednesday, the United Nations said it was looking into reports that the IS group 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," the spokesman for the U.N. secretary-general, Stephane Dujarric, told reporters. "These kinds of reports are disturbing, to say the least."

He said the U.N. and humanitarian partners will try to "fill in the gaps" to meet water needs for the affected population.

Earlier this year, the IS group reduced the flow through another lock outside the militant-held town of Fallujah. But the extremists soon reopened it after criticism from residents.

The IS group captured Ramadi, the provincial capital of Anbar, last month, marking its most significant advance since a U.S.-led coalition began an air campaign against the extremists last year.