Iraqi forces launched an operation on Tuesday to retake a northern town from the Islamic State group in the latest move to prepare a broad offensive on jihadist bastion Mosul.

Army and tribal forces pushed towards Sherqat, which IS fighters captured 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 in Salaheddin province, 260 kilometres (160 miles) northwest of Baghdad and around 80 kilometres (50 miles) south of Mosul.

IS overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led air strikes and training have since retaken significant ground including the cities of Ramadi and Fallujah.

Iraqi forces have already reconquered other towns north of Sherqat on the way to Mosul but the question of Shiite 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."

Colonel Mohammed al-Assadi, an Iraqi security spokesman, said the country's forces had retaken villages around Sherqat and were about five kilometres (three miles) from the town.

Colonel John Dorrian, the spokesman for the US-led operation against IS, said coalition forces had carried out 19 air strikes over the past two weeks to set the stage for the Sherqat operation.

Sherqat is "in close proximity to their supply lines," Dorrian said, referring to routes by which Iraqi forces move troops and material for operations against IS.

"Clearing that area makes sure that their supply lines are protected," he said.

Ahmed al-Assadi, the spokesman of the Hashed al-Shaabi (Popular Mobilisation) paramilitary forces, said operation "Sherqat Dawn" aimed to "finish expelling those terrorist gangsters from usurped Iraqi land."

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 Shiite militia.

- Obama confident -

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 Shiite 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.

Iraqi forces have been moving northwards from Baghdad for almost two years, gradually retaking areas over which IS declared its "caliphate" in June 2014.

The jihadists have also lost ground in Syria and Libya.

Iraqi forces 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 farther 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 (IS) operation in Mosul."