Iraq’s prime minister said early Monday (Iraq time) that its ground forces are starting the final offensive to drive the Islamic State from Mosul in what will be a defining battle in the campaign against the militant group.

Iraq’s security forces backed by U.S.-led coalition airstrikes will face several thousand militants in the sprawling city. They are surrounded by improvised explosives and other obstacles, as well as ditches filled with oil that could be set on fire.

Mosul, Iraq’s second largest city, is also home to about one million civilians, many of whom have been unable to flee the city.

Brett McGurk tweeted Sunday that Iraqi Prime Minister Haider al-Abadi issued the orders to start the offensive. “We are proud to stand with you in this historic operation,” he tweeted.

The U.S.-led coalition fighting the Islamic State extremist group has been stepping up airstrikes in and around the militant-held city of Mosul as thousands of Iraqi ground troops moved into position for a push to retake Iraq's second largest city, officials said.

The coalition said in a statement Friday that its planes conducted more than 50 airstrikes in the past two weeks around Mosul, which has shrunk from 2.5 million people to around 600,000 since falling under extremist control two years ago.

It is the last major Iraqi city still in the hands of the Islamic State, also known as ISIL or ISIS.

The plan to wrest control of the city, located 250 miles north of Baghdad, involves some 30,000 troops in one of the most complex operations yet for Iraq's military.

“The size of Mosul makes this by far the largest task the ISF has undertaken to date,” Air Force Col. John Dorrian, a coalition spokesman, said last week, referring to Iraqi security forces.

Contributing: Doug Stanglin