Iraqi forces pushed into the Islamic State group stronghold of Hawija on Wednesday and seized some territory, stepping up their assault against one of the jihadists' last enclaves in the country.
Government and allied forces backed by a US-led coalition launched an offensive last month to oust IS from Hawija, a longtime insurgent bastion.
The town is among the final holdouts from the territory seized by the jihadists in 2014 and its recapture would leave only a handful of remote outposts in IS hands.
The commander for the offensive, Lieutenant General Abdel Amir Yarallah, said the army, federal police and rapid response force had begun a major operation "to liberate the centre of Hawija and the neighbouring town of Riyadh".
Federal police chief Raed Shakir Jawdat said elite units had entered the town from the northwest amid artillery and missile bombardments of jihadist positions.
Two western neighbourhoods of the town were taken, and the immediate goal was to take five more, he said.
Besides the security forces, the operation also involves tribal volunteers and the Hashed al-Shaabi paramilitary force, mainly made up of Iran-trained Shiite militia.
The Hashed said its engineers were demining the route into Hawija and that IS fighters had retreated to the town centre after "their defences were breached".
- 12,500 flee offensive -
Security sources said government forces had retaken areas along the Tikrit-Kirkuk highway south of Hawija that had been under IS control for three years.
They had also entered the town of Riyadh southeast of Hawija.
The United Nations said on Tuesday that an estimated 12,500 people had fled the town since the launch of the offensive to retake Hawija and surrounding areas last month.
The UN's humanitarian affairs office said the number of people still in the town was unknown but could be as high as 78,000.
It said humanitarian agencies have set up checkpoints, camps and emergency sites capable of receiving more than 70,000 people who could flee.
Hawija, 230 kilometres (140 miles) north of Baghdad, is one of just two significant areas of Iraq still held by IS, along with a stretch of the Euphrates Valley near the Syrian border which is also under attack.
It has been an insurgent bastion since soon after the US-led invasion of 2003, earning it the nickname of "Kandahar in Iraq" for the ferocious resistance it put up similar to that in the Taliban militia's citadel in Afghanistan.
The town's mainly Sunni Arab population is deeply hostile both to the Shiite-led government in Baghdad and to the Kurds who form the historic majority in adjacent areas.
Hawija lies between the two main routes north from Baghdad -- to second city Mosul, recaptured from IS in July, and to the city of Kirkuk and the autonomous Kurdish region.
An army colonel said Wednesday that Iraqi troops had also retaken a power plant and a bridge north of Baghdad from the jihadists, meaning "there is no longer any IS presence in Salaheddin province".
IS has been forced out of most of the territory it seized in Iraq and Syria during a lightning offensive in the summer of 2014 that was followed by its declaration of a cross-border "caliphate".
The US-led coalition is also backing an Arab-Kurdish alliance, the Syrian Democratic Forces, that is battling to oust IS from its de facto Syrian capital Raqa.
- Under pressure in Iraq, Syria -
The SDF has captured about 90 percent of Raqa and is fighting fierce battles with remaining IS jihadists.
IS's other main stronghold in Syria is the eastern province of Deir Ezzor, which borders IS-held territory in Iraq.
Two separate offensives are under way against the jihadists in Deir Ezzor -- one by the SDF, the other by government forces supported by Russia.
The Syrian Observatory for Human Rights war monitor said a Russian air strike killed at least 20 civilians on Wednesday as they tried to cross the Euphrates river to escape fighting in Deir Ezzor province.
The Britain-based Observatory also said Syria's army and allied fighters had driven the jihadists from their final positions in the central province of Hama.
It said regime forces had taken control of "the last remaining villages" in IS hands in eastern Hama province, after more than a month of heavy fighting.
Analysts have said that as it comes under increasing pressure in Iraq and Syria, IS is likely to seek to carry out more attacks abroad.
The group this week claimed responsibility for the Las Vegas shooting massacre, but US officials have reacted cautiously and experts say IS may be trying to rally its supporters with false claims.