The number of U.S. troops diagnosed with mild traumatic brain injury since an Iranian airstrike earlier this month has increased to 64, the Pentagon said Thursday.

The figure has changed several times since the Jan. 8 missile attack on U.S. and coalition forces in Iraq. President Trump initially said no Americans were harmed in the Iranian retaliatory strike for the death of Quds Force Gen. Qassem Soleimani in Baghdad days earlier.

The increase comes days after the Pentagon updated the number of American military personnel from 34 to 50. Of those injured, 39 service members have returned to duty, the Pentagon said.

Defense officials warned the numbers could change as symptoms could develop later.

"These things are cumulative too," Army Gen. Mark Milley, chairman of the Joint Chiefs of Staff, said at a Thursday Pentagon news briefing. "So if you get multiple concussions that can manifest itself down the road. It could be a year, two years. We're early in the stage of diagnosis. We're early in the stage of therapy for these troops. We'll continue to monitor them – for the rest of their lives, actually."

Trump appeared to downplay the injuries suffered by U.S. troops.

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"I don’t consider them very serious injuries relative to other injuries that I’ve seen,” Trump told a reporter in Switzerland last week where he attended the World Economic Forum in Davos. “I’ve seen what Iran has done with their roadside bombs to our troops.”