The army's 7th division, police and fighters from local tribes aim to liberate towns still under IS control in Anbar province, backed by air strikes by the US-led international coalition.
"A military operation has begun in the western areas of Anbar to liberate them from Daesh," said Lieutenant General Qassem Mohammedi, commander of the Jazeera Operations Command, using an Arabic acronym for IS.
Five thousand troops and more than 2,000 tribal fighters are engaged in the assault, an Iraqi army officer told The New Arab's Arabic publication.
The main targets of the operation are Anah, Rawa and al-Qaim, the westernmost Iraqi towns along the Euphrates Valley.
The militant hub of al-Qaim, which lies 330 kilometres (200 miles) northwest of Baghdad, is still a long way down the road and the most immediate target of the new offensive is the town of Anah.
"Our forces started advancing from Haditha towards Anah from several directions," Mohammedi said.
Haditha was never seized by IS when the group swept across much of Iraq's Sunni Arab heartland in 2014 and is home to a tribe that has led the fight against militants in the area.
"Zero hour has come to liberate the western areas," Nadhom al-Jughaifi, a commander with the Haditha tribal fighters, said.
Anbar is a desert area that borders Saudi Arabia, Jordan and Syria. Security in reconquered areas remains precarious as extemists continue to move across the province.
Tens of thousands of Iraqi forces are currently involved in an offensive to retake Mosul, the main northern city which is also IS's last major stronghold in the country.
IS has lost more than half of the territory it once controlled in Iraq and the loss of Mosul would deal a major blow to the "caliphate" it proclaimed there in June 2014.