50,000 IS militants killed since 2014

"I am not into morbid counts but that kind of volume matters, that kind of impact on the enemy,” the official said, adding that the figure was "conservative."

If accurate, the figure would be higher than estimates from August when Lt. Gen. Sean MacFarland said about 45,000 combatants had been taken off the battlefield.

US officials have been reluctant to disclose specific numbers and have said IS is able to replenish its ranks rapidly.

"I give them credit for being so resilient,” the official said, Fox News reported.

Air Force Col. John Dorrian, a spokesman for the US military in Iraq, told AP that the ongoing military campaign is reducing the numbers and capabilities of IS.

"Unfortunately were seeing younger fighters; perhaps adolescent age, rather than adults," Dorrian told the news agency AP.

"What that tells us is they're beginning to run out of those resources," Dorrian said. "It doesn't mean that it's not still an extraordinarily dangerous situation. They are not going to go quietly, but they are going to go."

What next for Mosul?

A key issue now is what forces may be needed to help Iraqi troops hold Mosul once Islamic State fighters are pushed out of the city.

Seven weeks into the operation to retake Mosul, Iraqi fighters have been pushing through the eastern section of the city, but have faced fierce resistance from IS insurgents who took control of the city two years ago.

Since the launch of Operation Inherent Resolve in August 2014, over 125,000 sorties in Iraq and Syria have been carried out by the US-led coalition, the Pentagon said in November.

jbh/bw (AP, Reuters)