The U.S.-led coalition has killed 50,000 Islamic State fighters in the last two years in Iraq and Syria, marking a milestone in the long terror fight, a senior U.S. military official told Fox News on Thursday.

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The official made the remarks for the first time and called the figures a conservative estimate, but said it was more than what others have stated before.

U.S. leaders have expressed reluctance to disclose specific numbers, and noting that ISIS had been able to replace fighters rapidly, particularly early on.

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Speaking about ISIS, the official said, “I give them credit for being so resilient.”

The official warned airstrikes have killed so many ISIS fighters, fears remain that the terror group could go underground and morph into something he called “AQI 2.0,” or another version Al Qaeda in Iraq.

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

The official said 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.

The U.S.-led coalition has flown more than 125,000 sorties in Iraq and Syria since Operation Inherent Resolve began in Aug. 8, 2014, the Pentagon reported in November.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.