Iraqi Prime Minister Haider al-Abadi announced the start of military operations to liberate the northern city of Mosul from Islamic State militants on Monday, launching the country on its toughest battle since American troops left nearly five years ago, according to The Associated Press (AP).

State TV reportedly aired a brief statement in the early hours Monday announcing the start of the widely anticipated military offensive to drive IS out of Iraq’s second-largest city.

Broadcasts showed the prime minister, dressed in the uniform of the elite counterterrorism forces, speaking while flanked by senior military officers.

“I announce today the start of the heroic operations to free you from the terror and the oppression of Daesh,” Abadi said in a speech on state TV, using an Arabic acronym for Islamic State.

Qatar-based al-Jazeera television aired video of what it said was a bombardment of Mosul that started after Abadi’s speech, showing rockets and bursts of tracer bullets across the night sky and loud sounds of gunfire.

The assault on Mosul, with a population of 1.5 million, is backed by the U.S.-led coalition, which is providing air and ground support.

The offensive could be one of the biggest military operations in Iraq since the 2003 U.S.-led invasion that toppled Saddam Hussein and the biggest mounted by the Iraqi government since U.S. military forces withdrew in 2011.

About 30,000 troops were expected to take part from the Iraqi army, Kurdish Peshmerga militia and Sunni tribal fighters, while estimates of Islamic State forces in the city range from 4,000 to 8,000.

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 the Islamic State. A statement on Al-Abadi’s website pledged the fight for the city marked a new phase that would lead to the liberation of all Iraqi territory from the militants this year.

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.