The US has launched a series of air strikes against Isis (known as Islamic State) militants who are attempting to seize control of the strategically vital Haditha dam from the Iraqi army.
The strikes were the first launched by US forces in Anbar province, after the US government said that operations would be limited until a fully representative government had been formed in Baghdad.
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 [Islamic State] terrorists near the Haditha dam."
The Pentagon's press secretary, Rear Admiral John Kirby, later said: "We conducted these strikes to prevent terrorists from further threatening the security of the dam, which remains under control of Iraqi security forces, with support from Sunni tribes."
The outgoing Iraqi government requested US air support against Islamic State in all areas attacked by the militant group. The US had agreed to meet the request only after Iraq's Sunni population is fully represented in the new government.
Outgoing Iraqi prime minister Nouri al Maliki is accused of running a sectarian Shia government, which alienated the country's Sunni population, leading many to support Islamic State as it seized swathes of Iraq.
Islamic State advances
After the militant group threatened to overrun Kurdistan province and wipe out the country's Yazidi minority, the US launched a series of air strikes, with 130 attacks having been launched since early August.
They have helped drive Islamic State fighters from Iraq's largest dam at Mosul.
The strikes near Haditha open a new front in US air operations in Iraq.
IS currently controls Fallujah dam on the Euphrates river, but have been prevented seizing the country's second largest dam in Haditha by government forces supported by Sunni tribes.
Control of the dams could allow the group to flood areas of Iraq, and control electricity supplies.
When Iraqi forces moved into Fallujah, IS flooded land downriver.
Water is a precious resource in Iraq, which is mainly composed of deserts.
Lower water levels in the Euphrates have caused electricity shortages south of Baghdad in recent years, where towns rely on water operating generators for electricity.