The US launched four airstrikes against Islamic State insurgents near Iraq's Haditha Dam early Sunday to prevent its capture and potential exploitation by the Sunni militants, officials said.

Sunday's strikes represent the first expansion of the US air campaign into Iraq's western Anbar province since the bombing began in early August. The move brings Washington's military operations even closer to the Syrian border.

Defense Secretary Chuck Hagel said the strikes were carried out at the request of the Iraqi government, whose forces, together with a band of Sunni militia, have been battling IS insurgents in Anbar since January.

"If that dam would fall into ISIL's (Islamic State's) hands or if that dam would be destroyed, the damage that that would cause would be very significant and it would put a significant, additional and big risk into the mix in Iraq," Hagel told a news conference from Georgia, where he is meeting with government and defense officials.

The cooperation between the Sunni militia and Shi'ite-led government is an uncommon show of partnership to prevent Sunni Islamic State militants from overrunning the desert province bordering the Euphrates River.

A pro-Iraqi government paramilitary leader told Reuters the strikes took out a militant patrol attempting to raid the country's second largest hydroelectric dam, which is a primary source of water and electricity for millions of Iraqis.

"They (the air strikes) were very accurate. There was no collateral damage ... If Islamic State had gained control of the dam, many areas of Iraq would have been seriously threatened, even Baghdad," Sheik Ahmed Abu Risha said.

Five Islamic State Humvees, another armed vehicle, a checkpoint, and a militant bunker were all destroyed in the strike, the US military said.

"We will continue to conduct operations as needed in support of the Iraqi Security Forces and the Sunni tribes, working with those forces securing Haditha Dam," Pentagon Press Secretary Rear Admiral John Kirby said in a statement.

"The potential loss of control of the dam or a catastrophic failure of the dam — and the flooding that might result — would have threatened US personnel and facilities in and around Baghdad, as well as thousands of Iraqi citizens," Kirby said.

IS militants have been seizing strategic assets and critical infrastructure such as oil fields, dams, and power plants during a lightning advance across Iraq's north and west this summer.

In April, the group fought and gained control of the Fallujah Dam on the Euphrates River. They later used it to flood areas downriver outside of Baghdad as government forces closed in on the militants.

Last month, the IS also managed to seize Mosul Dam in the north of Iraq before government forces assisted by US airstrikes recaptured it. Continued strikes are reportedly helping to fend off militant attacks.

The US confirmed a fresh offensive against insurgents at Mosul on Sunday, bringing the total number of airstrikes to 133.

Hagel denied that the move indicated an escalation of the US operations in the region, but said that the government continues to explore its options in relation to targeting the IS at its core bases in Syria.