The operation came a day after IS 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".
"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.
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Yarallah said today'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.
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.
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 (IS) 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 IS, along with a stretch of the Euphrates Valley near the Syrian border which is also under attack.
The international coalition against IS also said that US-led air strikes in Iraq and Syria have killed another 50 civilians, without specifying when.
It said that with the latest deaths, "at least 735 civilians have been unintentionally killed by coalition strikes".
Further up the Euphrates Valley on the Syrian side of the border, IS is facing rival offensives by US-backed fighters and Russian-backed government forces.
The jihadists launched a major counteroffensive against government forces yesterday, killing at least 73 troops and militia in a series of attacks along their supply lines, the Syrian Observatory for Human Rights said.
Syrian troops pushed through the desert and broke a three-year IS siege of government enclaves in Deir Ezzor earlier this month. They are now battling to retake the remaining IS posts.
Further upstream, a US-backed alliance of Kurdish and Arab fighters is poised to capture the onetime IS bastion of Raqa, once a byword for jihadist atrocities.
A top US-led coalition commander told AFP yesterday that the jihadists were now breathing their "last gasps" in the city.