Iraqi Prime Minister Haider al-Abadi announced late Sunday the start of a military offensive to retake Mosul, the country's second largest city, from the Islamic State group.

State TV showed a brief written statement announcing the start of the widely anticipated military offensive to drive the terror group out of the northern city home to more than a million civilians.

Broadcasts showed the prime minister, dressed in a military uniform, speaking while flanked by senior officers. State TV broadcast patriotic music within minutes of the announcement.

The thuds of sporadic artillery shelling rumbled across the rolling Nineveh plains in the direction of Mosul, witnesses said.

"This is a decisive moment in the campaign to deliver ISIL a lasting defeat," said Secretary of Defense Ash Carter in a statement. "The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead. We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL's hatred and brutality."

Mosul fell to the Islamic State group in June of 2014, when the extremist group blitzed across northern and western Iraq, overrunning nearly a third of the country and plunging Iraq into its most critical political and security crisis since the U.S.-led invasion in 2003.

"Godspeed to the heroic Iraqi forces, Kurdish Peshmerga, and Ninewa volunteers," Brett McGurk, US Envoy to the coalition against ISIS, said in a tweet. "We are proud to stand with you in this historic operation."

Tonight, PM Abadi issued orders to initiate major operations to liberate #Mosul after two years of darkness under #ISIL terrorists. #الموصل — Brett McGurk (@brett_mcgurk) October 16, 2016

Godspeed to the heroic Iraqi forces, Kurdish #Peshmerga, and #Ninewa volunteers. We are proud to stand with you in this historic operation. — Brett McGurk (@brett_mcgurk) October 16, 2016

The push to retake Mosul will be the biggest military operation in Iraq since American troops left in 2011 and, if successful, the strongest blow yet to ISIS.

Iraqi forces have been massing around the city in recent days, including members of the elite special forces, who are expected to lead the charge into the city itself.

After seizing Mosul, ISIS leader Abu Bakr al-Baghdadi visited the city to declare an Islamic caliphate that at one point covered nearly a third of Iraq and Syria.

But since late last year, the militants have suffered battlefields losses in Iraq and their power in Iraq has shrunk to Mosul, about 225 miles northwest of Baghdad, along with small towns in the country's north and west.

The operation to retake Mosul is expected to be the most complex yet for Iraq's military, which has been rebuilding from its humiliating 2014 defeat.

Iraqi forces began moving into Nineveh province to surround Mosul in July, when ground troops led by the country's elite special forces retook Qayara air base south of the city.

Thousands of Iraqi troops were deployed there ahead of the planned operation. Iraqi troops were also positioned east of Mosul in the Khazer area, along with Kurdish Peshmerga forces, and to the north of the city near the Mosul Dam and Bashiqa areas.

Before the prime minister's announcement, Brig. Gen Haider Fadhil told The Associated Press in an interview that more than 25,000 troops, including paramilitary forces made up of Sunni tribal fighters and Shiite militias, will take part in the offensive that will be launched from five directions around the city.

In addition to carrying out airstrikes, the U.S.-led international coalition will also offer artillery fire, he added.

The role of the Shiite militias has been particularly sensitive, as Nineveh is a majority Sunni province and Shiite militia forces have been accused of carrying out abuses against civilians in other operations in majority Sunni parts of Iraq.

According to U.N. estimates, up to 1 million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country. The conflict in Iraq has forced 3.3 million people to flee their homes, with most of them living in camps or informal settlements.

Fadhil voiced concern about potential action from Turkish troops based in the region of Bashiqa, northeast of Mosul. Turkey sent troops to the area late last year to train anti-ISIS fighters there.

But Baghdad has seen the Turkish presence as a "blatant violation" of Iraqi sovereignty and has demanded the Turkish troops withdraw, a call Ankara has ignored.

Fox News' Lucas Tomlinson The Associated Press contributed to this report.