At least 50,000 Islamic State terrorists have been killed by the US-led coalition since it began operations in Iraq and Syria in late 2014, a senior US military official said Thursday.

“I am not into morbid counts but that kind of volume matters, that kind of impact on the enemy,” the official said.

The official, who spoke to Pentagon reporters on condition of anonymity, said the number was a “conservative estimate,” it’s a bit more than what others have stated before. US leaders have expressed reluctance to disclose specific numbers, and note that IS has been able to replace fighters rapidly, particularly early on.

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“I give them credit for being so resilient,” the official said according to Fox News.

In August, Lt. Gen. Sean MacFarland said about 45,000 combatants have 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.

Since the launch of Operation Inherent Resolve began on August 2014, more than 125,000 sorties in Iraq and Syria have been carried out by the US-led coalition, the Pentagon said last month.