"At the request of the government of Iraq, US military forces attacked ISIL (Islamic State) terrorists near Haditha in Anbar province in support of Iraqi security forces and Sunni tribes protecting the Haditha dam," the US Central Command said in a statement.
"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," Pentagon spokesman Rear Admiral John Kirby said in a statement.
It is the first time that Washington has carried out air strikes in support of Sunni Arab tribal militia since it launched its air campaign in Iraq on August 8.
Previous strikes has been mainly in support of Kurdish forces in the north, although late last month it gave limited air support to the army and Shiite militia further south in breaking an IS siege of the Shiite Turkmen town of Amerli.
There have been repeated attempts by IS to take Haditha Dam.