The US is carrying out a series of air strikes on Islamic State militants close to the vital Haditha dam in western Iraq, US officials say.
The US strikes, the first in the area, are to protect the Iraqi forces and Sunni tribesmen in control of the dam.
The US has carried out scores of air strikes to support Iraqi and Kurdish forces as they try to halt the advance of the militants in the north and west.
Earlier, Kurdish forces retook the strategically important Mount Zarta.
A US official said: “At the request of the Iraqi government and in keeping with our mission to protect US personnel and facilities, US military planes have begun striking Isil terrorists near the Haditha Dam.
“These strikes are being conducted to prevent the terrorists from further threatening Iraqi security forces in control of the dam.”
IS is also often referred to as Isil or Isis.
The US has now carried out more than 130 air strikes since early August.
Islamic State fighters have targeted a number of dams in their offensive, capturing the facility at Fallujah.
They also took the largest dam, at Mosul, but US air strikes helped force them out.
The group has so far failed in its attempts to capture Haditha dam in western Anbar province. It is Iraq’s second largest dam.
IS militants in August reportedly closed eight of the Fallujah dam’s 10 lock gates that control the river flow, flooding land up the Euphrates river and reducing water levels in Iraq’s southern provinces, through which the river passes.
Many families were forced from their homes and troops were prevented from deploying, Iraqi security officials said.
IS also controls other key national assets – several oil and gas fields in western Iraq and Syria.