For months, the Iraqi government has been preparing to launch an offensive to recapture the country’s second largest city, Mosul, from the Islamic State. Now, there are clear signs that the militants believe it is imminent.

The extremist group’s fighters have sent their wives and children to Syria and Turkey, pulled their black flags from buildings to hide potential airstrike targets for a U.S.-led coalition, planted roadside bombs and set fires to oil wells to stymie Iraqi troops.

The Islamic State “has announced its highest alert,” said Arm Khaled al Obaidi, a Mosul resident and former policemen. “They canceled all leave and demanded fighters on leave return to duty.”

This month, Iraqi forces recaptured from the militants an airstrip about 40 miles south of Mosul that  will serve as an important staging area for Iraqi troops. At the same time, the U.S. military said it is sending an additional 560 troops to Iraq for duties that include guarding the air base in anticipation of the long-promised siege.

Mosul is the Islamic State’s last power center in the country. Iraqi forces already recaptured Fallujah — about 40 miles west of Baghdad — and other cities that the militant group had controlled since invading the country two years ago.

Iraqi Prime Minister Haider al-Abadi pledged to retake Mosul before the end of the year, but it’s not clear when an offensive could begin in earnest.

Mosul resident Faisal Essawi said now is the time to confront the Islamic State because some fighters are deserting the group in anticipation of an epic battle.

“The government must take the opportunity of the chaos (the Islamic State) is suffering after losing the airbase,” Essawi said. “Some of their leaders have gone missing. The organization is struggling to fill the gap.”