The military last week said it had conducted follow-on screenings and that the number of troops who were pulled out of Iraq for brain injury treatment was 11. On Friday, Hoffman told reporters that, of the 34 troops diagnosed with brain injuries, 17 have returned to duty, including one service member who was transferred to Kuwait for treatment.

Eight service members have been transferred to the U.S. for treatment after initially being sent to Germany. Nine are still undergoing treatment and evaluation in Germany.

Hoffman attributed a change in numbers to symptoms that “are late-developing” and “manifest over a period of time.”

“What we saw was a number of people who were initially screened for concussion-like symptoms … saw their conditions improve rapidly, and then others, we saw their conditions didn’t improve,” Hoffman said. “Some got worse and some had severe enough symptoms that they were transported on for further treatment.”

Trump on Wednesday played down the initial reports of injuries.

“I heard that they had headaches, and a couple of other things,” Trump told reporters at the World Economic Forum in Davos, Switzerland. “But I would say, and I can report, it is not very serious. Not very serious.”

Esper on Wednesday told reporters that the Pentagon doesn't normally report such injuries. "This is mostly outpatient stuff. So we can track that if — if you're really interested in it."

He also said at the time he didn't know how many were injured. "I'm not a doctor and I'm not the ones evaluating them," he said.

Speaking Friday at the Center for Strategic and International Studies, Esper defended the Pentagon’s reporting of injuries around the globe.

“We’re fully committed to being transparent about what happens, but we need to make sure we’re accurate and that we categorize things properly, and that’s our commitment,” Esper said. “And it’s not just Iraq. It’s wherever troops are engaged.”