"It's not a good metric of success and it's not an easy number to determine," Cook told reporters. "We don't have great confidence in that number [of 50,000 killed]. I don't know if it's right or wrong."
On December 9, CNN cited a senior US military official, speaking on condition of anonymity, as saying at least 50,000 Daesh militants had been killed since the US-led coalition operations against the group began. The unnamed official called the estimate conservative.
However, Cook said it was very difficult to establish the exact numbers of militants killed, especially from air strikes.
"We don't necessarily have the ability to assess everything on the ground in the way we might like," he noted.
Cook said the US government remained committed to bringing every available pressure to bear to destroy Daesh.
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