The Iraq government on Monday launched a major offensive to drive the Islamic State group out of its stronghold of Mosul. The campaign is being seen as the country’s the biggest since United States forces left its soil in 2011.

Iraq’s Prime Minister Haider al-Abadi told Iraqi citizens through state television, “I announce today the start of the heroic operations to free you from the terror and the oppression of Daesh [a local term for the militant group]”. “We will meet soon on the ground of Mosul to celebrate liberation and your salvation,” Abadi added.

According to Al Jazeera, preparations for the offensive had begun and forces have surrounded the city. At least 30,000 troops are expected to fight for the Iraqi government, while the Islamic State group has 4,000-8,000 fighters in the city, Reuters reported. This coalition includes fighters from the Kurdish Peshmerga and Sunni tribal groups.

The US-led coalition that backs the Iraq government is expected to launch airstrikes in the area, even as 1.5 million people continue to live in the city under the Islamic State’s control. The United Nations said the offensive is a cause for “extreme concern” for the civilians living in the city, who are likely to be trapped as the strikes begin. The Islamic State group has been known to use citizens as human shields in the past.

Mosul is Iraq’s second-biggest city and has been under the control of the militant group since 2014. The US claimed that if the forces could defeat the militants in Mosul, it would deal a lasting blow to its influence in the region.