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.