The Pentagon has said that 34 US troops were diagnosed with traumatic brain injuries (TBI) following an Iranian attack on their base in Iraq.
Seventeen troops are still under medical observation, a spokesman said.
President Donald Trump had said no Americans were injured in the 8 January strike, which came in retaliation for the US killing of an Iranian general.
Mr Trump had cited the supposed lack of injuries in his decision not to strike back against Iran.
But last week, the Pentagon said 11 service members had been treated for concussion symptoms from the attack.
Asked about the apparent discrepancy this week at the World Economic Forum in Davos, Switzerland, Mr Trump said: “I heard that they had headaches, and a couple of other things, but I would say, and I can report, it’s not very serious.”
“I don’t consider them very serious injuries relative to other injuries I have seen,” he said when asked about possible TBIs.
The Pentagon says no Americans were killed in the Iranian missile strike on the Ain al-Asad base, with most sheltering in bunkers as missiles rained down.
On Friday, defence department spokesman Jonathan Hoffman told reporters that eight of the affected soldiers have been sent back to the US for further treatment, while another nine are being treated in Germany.
Sixteen troops were treated in Iraq and one in Kuwait before all 17 were returned to active duty, officials say.
Mr Hoffman added that the US Defence Secretary Mark Esper had not immediately been aware of the injuries in the days after the attack.
Iraq & Afghanistan Veterans of America, a non-profit organisation, slammed the Trump administration for taking so long to reveal the extent of casualties.
“This is a big deal,” its founder Paul Rieckhoff tweeted. “The American people must be able to trust the government to share information about our sons and daughters in harms way. Nothing is more serious and sacred.”
TBIs are common in warzones, according to the US military.
The most common cause of a TBI for deployed soldiers is an explosive blast, writes the US Defense and Veterans Brain Injury Center.
They are classified as mild, moderate, severe or penetrating. A mild TBI is also known as a concussion, and can be caused by a blast’s “atmospheric over-pressure followed by under-pressure or vacuum”.
The air vacuum is capable of penetrating solid objects, making it possible for soldiers to avoid blunt force trauma but still receive an invisible brain injury.
On Friday, tens of thousands of Iraqis protested in the streets Baghdad against the presence of some 5,000 foreign troops in the country.
The Iraqi parliament has urged all foreign fighters – including from the US – to leave.