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

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)

"Tonight, PM Abadi issued orders to initiate major operations to liberate #Mosul after two years of darkness under #ISIL terrorists," Brett McGurk, special presidential envoy for the Global Coalition to Counter ISIL, tweeted.