WASHINGTON -- A senior U.S. military official for the first time says the U.S.-led coalition has killed 50,000 militants with the Islamic State of Iraq and Syria, or ISIS, in the last two years in Iraq and Syria.

The official said it was a conservative estimate, but it’s a bit more than what others have stated before. U.S. leaders have expressed reluctance to disclose specific numbers, and note that ISIS has been able to replace fighters rapidly, particularly early on.

In August, Lt. Gen. Sean MacFarland said about 45,000 combatants have been taken off the battlefields.

The official says coalition airstrikes could be more aggressive in places like Mosul, where Iraqi troops are battling to retake the city, but civilian casualties are a risk. The official wasn’t authorized to discuss the matter publicly so spoke on condition of anonymity.

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The official said the U.S. was concerned about ISIS fighters going into hiding, CBS Radio News correspondent Cami McCormick reports.

“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 said, referring to another name for ISIS, the Islamic State of Iraq and the Levant. “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.’”