Iraqi forces push into Islamic State stronghold of Hawija

"With God's help, the army, the federal police and the rapid response force began a major operation on Wednesday to liberate the centre of Hawija and the neighbouring town of Riyadh," Lieutenant General Abdel Amir Yarallah said in a statement.

Iraqi government forces and the Hashd al-Shaabi, an alliance of Shia militias, are fighting to retake the northern town of Hawija after expelling IS from nearly all of the territory the militants seized in Iraq in 2014.

The latest operation involves federal police, elite units, tribal volunteers and the army.

Police chief Raed Shakir Jawdat said that the latest "phase of the operation to liberate Hawija" had begun with artillery and missile fire on IS positions.

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He said that elite federal police units had entered the city from the northwest.

"They are advancing and the goal is to take seven neighbourhoods of Hawija and 12 vital objectives," he said, without providing more details. 

The United Nations said on Tuesday that an estimated 12,500 people had fled the town since the launch of the offensive last month.

The UN's humanitarian affairs office (OCHA) said the number of people still in the town was unknown but could be as high 78,000.

Hawija, about 230 kilometres north of Baghdad, is one of just two areas of Iraq still held by IS, along with a stretch of the Euphrates Valley near the Syrian border which is also under attack.