The U.S.-led coalition has killed an estimated 50,000 Islamic State fighters since President Barack Obama first ordered military action against the group in August 2014, according to a senior military official.
The official, speaking on condition of anonymity, told reporters the number was considered a "conservative estimate."
A second senior official told VOA the 50,000 were killed specifically in Syria and Iraq, and did not include the total number of IS fighters killed in other countries, such as Libya and Afghanistan.
The estimate was given to demonstrate how effective U.S.-led coalition airstrikes have been over the past two years.
The devastating number comes as Col. John Dorrian, the coalition's spokesman in Iraq, said Islamic State forces in Mosul are losing fighters at an "unsustainable rate" because they are "completely surrounded without the ability to reinforce."
"They [Islamic State] are not going to go quietly, but they are going to go," Dorrian told reporters Thursday.
He said counter-IS forces have seen the terror group using younger fighters of adolescent age as its numbers start to fall in Mosul, a tactic he called "unconscionable."
A senior military official at the Pentagon told reporters the group's dwindling numbers likely will cause the group to transition to what he called "AQ-I 2.0," which stands for the next update of al-Qaida in Iraq.
"Sooner or later they're just going to go to ground here and try to stay relevant, in terms of the ideology, through external attacks and terrorist activities," the official said.
He said the coalition is currently working to stay ahead of this developing trend.