Iraqi forces launched an operation on Tuesday to retake a northern town from the IS group in the latest move to prepare a broad offensive on jihadist bastion Mosul.
Army and tribal forces pushed towards Sherqat, a town IS militants took more than two years ago when they swept across Iraq's Sunni Arab heartland.
The town lies on the west bank of the Tigris river 260 kilometres (160 miles) northwest of Baghdad and around 80 kilometres (50 miles) south of Mosul.
Iraqi forces have already reconquered other towns north of Sherqat on the way to Mosul but the question of Shia militia involvement in military operations there had held up the push.
"The operation to liberate Sherqat started at 5:30 am (0230 GMT) from several directions... with the support of coalition forces," Joint Operations Command spokesman Yahya Rasool said.
"We are making good progress," he told AFP. "Sherqat is important, we can't move on Mosul and have terrorists control Sherqat."
Sherqat lies in the far north of Salaheddin province, which includes the cities of Samarra and Tikrit, and close to the border with Nineveh province of which Mosul is the capital.
Ahmed al-Assadi, the spokesman of the Hashed al-Shaabi (Popular Mobilisation) paramilitary force, also announced the operation.
"The sons of Hashed al-Shaabi and the Iraqi army backed by the air force launched the 'Sherqat Dawn' operation to finish expelling those terrorist gangsters from usurped Iraqi land," he said.
The Hashed al-Shaabi, which has played a big part in retaking IS-held areas since 2014, is nominally under the control of the prime minister but dominated by Tehran-backed Shia militia.
It also includes less powerful Sunni tribal forces supporting the government against IS.
Rasool stressed that only tribal forces -- sometimes referred to as Hashed al-Ashaeri (Tribal Mobilisation) -- were fighting in Sherqat, not Shia militias.
Prime Minister Haider al-Abadi, speaking from New York where he met US President Barack Obama, said the same operation also included efforts to flush out IS fighters from desert areas near Ramadi and Heet in the western province of Anbar.
While most towns and cities in Anbar are now under government control, IS fighters are still able to move across parts of the vast arid province and have continued to harass Iraqi forces.
Hashed al-Shaabi forces had vowed they were days away from Sherqat almost a year ago, when they tried to keep their momentum after retaking the town of Baiji and pushed on northwards.
They stopped advancing however when air strikes by the US-led coalition, which has been reluctant to support groups loyal to Iran that have attacked US interests in the past, also stopped.
Popular Mobilisation forces took significant casualties in recent fighting against IS in Jazirat al-Khaldiyeh, an area in Anbar that was a major crossroads for fighters and supplies moving across Iraq.
Iraqi forces have been moving northwards from Baghdad for almost two years, gradually retaking areas over which IS declared its "caliphate" in June 2014.
They have left some pockets of IS jihadists on the way however -- such as in Hawijah or in the Hamreen mountains -- and priority was given to Qayyarah, a town further north which will be used as a launchpad for an offensive on Mosul.
Speaking before talks with Abadi, Obama said he could see quick progress in the battle for Mosul, the last major IS stronghold in Iraq since government forces retook Fallujah in June.
"We feel confident that we will be in a position to move forward fairly rapidly," Obama said, vowing to fight "right at the heart of the ISIL operation in Mosul."
The United States alone now has 4,460 troops in the country, backed by hundreds more from Western allies, advising and assisting Iraqi government and Kurdish forces.