The operation came after ISIS released what it said was an audio recording of its leader Abu Bakr al-Baghdadi urging resistance, the first such intervention in nearly a year.
“The leaders of the Islamic State and its soldiers have realised that the path to… victory is to be patient and resist the infidels whatever their alliances,” said the voice in the recording, whose authenticity Washington said it had “no reason to doubt”.
Since Baghdadi’s previous message to his followers last November, the territory the jihadists still hold in the cross-border caliphate they proclaimed in 2014 has shrunk to a fraction of its former extent.
“A huge military operation has begun to liberate Hawija and its surrounding areas,” the operation’s commander, Lieutenant General Abdel Amir Yarallah, said in a statement.
Iraqi forces launched an offensive to retake the jihadist enclave around Hawija on September 21, swiftly taking the town of Sharqat on its second day before pushing on towards Hawija itself.
Yarallah said that Friday’s assault marked the second phase of the operation and aimed to recapture Hawija and the towns of Al-Abbasi, Riyadh and Rashad to its west, east and south.
All are mainly Sunni Arab towns that have long been bastions of insurgency and were bypassed by government forces in their push north on second city Mosul last year which culminated in the ISIS’ defeat in their most emblematic stronghold this July.
Yarallah later announced that troops had taken Al-Abbasi and raised the Iraqi flag there.
He said the operation involved the army, the federal police, counterterrorism units and the Rapid Intervention Force, as well as tribal volunteers and the paramilitary Popular Mobilisation force, mainly made up of Iran-trained Shiite militia.
The enclave lies east of the Tigris River and south of one of its major tributaries, the Little Zab, and troops erected pontoon bridges during the night to enable the assault to begin, Yarallah said.
The Popular Mobilisation force said that ISIS had set fire to two oil wells in the Alas field, southeast of Hawija, in a bid to provide cover and slow the advance of loyalist forces.
Prime Minister Haider al-Abadi hailed the second phase of the operation to recapture the area.
“As we promised the sons of our country, we are going to liberate every inch of Iraqi land and crush the Daesh (ISIS) terrorist gangs,” Abadi said.
“We are on the verge of a new victory to liberate the residents of these areas from those criminals.”
The Hawija enclave is one of just two areas of Iraq still held by ISIS, along with a stretch of the Euphrates Valley near the Syrian border which is under attack too.
Deadly counterattack in Syria
Further up the Euphrates Valley on the Syrian side of the border, ISIS is facing rival offensives by US-backed fighters and Russian-backed government forces.
The ISIS launched a major counteroffensive against government forces on Thursday, killing at least 73 troops and militia in a series of attacks along their supply lines, the Syrian Observatory for Human Rights said.
Most of the dead came near the desert town of Sukhna, on the main highway between the big cities of the west and Euphrates Valley city of Deir Ezzor, the Britain-based monitoring group said.
Syrian troops pushed through the desert and broke a three-year ISIS siege of government enclaves in Deir Ezzor earlier this month. They are now battling to retake the rest of it.
Further upstream, a US-backed alliance of Kurdish and Arab fighters is poised to capture the onetime ISIS bastion of Raqa, once a byword for jihadist atrocities.
A top US-led coalition commander told AFP on Thursday that the jihadists were now breathing their “last gasps” in the city.
He said the coalition was already setting its sights on another ISIS-held town in the Euphrates Valley — Al-Mayadeen, between Deir Ezzor and the Iraqi border.