At least 50,000 fighters belonging to the Islamic State group, also called ISIS, have been killed by U.S.-led coalition forces in the last two years in which coalition troops have been battling the terrorist group in Iraq and Syria, a U.S. military official said.

The official, who wasn’t authorized to speak publicly on the matter, told a small group of reporters, the figure was a “conservative estimate.” However, the U.S. has warned that ISIS can replace fighters quickly.

The official added that coalition airstrikes may be more aggressive in places like Mosul where Iraqi forces are fighting ISIS to liberate the city from its clutches. The U.S., the official said, is concerned about fighters going into hiding.

“These are the guys that went to ground before when they were under a lot of pressure in the 2009-2010 time frame. Literally went to ground in the sanctuary of eastern Syria and western Iraq - Anbar, Ninewah - and then in two years’ time came back manifested as ISIL,” the official reportedly said, using another name for the militant group.

“They are more than skilled at ‘we’re under a lot of pressure right now, guys, let’s go low. Let’s outlast this thing and come back up,’” the official added.

Coalition forces are treading cautiously in their battle for Mosul and the Syrian city of Raqqa given the likelihood of civilian casualties. U.S.-led coalition forces have killed 173 civilians since starting their 2014 operations against ISIS in Iraq and Syria.

The militant group is still confident of emerging victorious despite losing ground and fighters, a new ISIS spokesperson said. Abulhassan al-Muhajer, in an audio statement Monday, vowed a “victory” for ISIS and urged fighters “not to flee.”