"These are mainly outpatient treatments", chief Pentagon spokesman Jonathan Hoffman told reporters, cautioning that the number of troops suffering as a result of the missile strikes could change.
Traumatic brain injuries are not always apparent immediately after they've been suffered.
Hoffman said that of the 34 with TBI, 18 were evacuated from Iraq to USA medical facilities in Germany and Kuwait, and 16 stayed in Iraq.
In the wake of the USA killing of Iran's Gen. Qassem Soleimani and Iran's retaliation in Iraq, President Trump has called on North Atlantic Treaty Organisation to contribute more forces to the anti-ISIS mission, though leaders have said no moves are underway to build up or draw down US forces in the country.
Pentagon officials have said there had been no effort to minimize or delay information on concussive injuries, but its handling of the injuries following Tehran's attack has renewed questions over the United States military's policy regarding how it deals with suspected brain injuries. Since then, officials had repeatedly declined to provide updates.
Asked Wednesday if he knew of any serious injuries among US personnel at Ayn al-Assad during the Iranian missile strike, Trump said he "heard that they had headaches, and a couple of other things".
"I heard that they had headaches, and a couple of other things", Trump said. He later characterized the injuries as "not very serious". "I've seen people with no legs and no arms".
Seventeen of the 18 evacuees were sent to Germany and nine remain there, according to Hoffman. Meanwhile, one service member was taken to Kuwait for treatment and has returned to duty. In total, 16 were treated on site in Iraq, one was transported to Kuwait for treatment and has since returned to duty, while 17 were flown to US facilities in Landstuhl, Germany.
Hoffman on Friday maintained that the military was taking the injuries seriously and said USA medical personnel were carefully tracking any troops who reported symptoms of a traumatic brain injury or concussion, including headaches, dizziness, sensitivity to light or nausea.
On Friday morning, Defense Secretary Mark Esper directed the Pentagon's acting undersecretary of defense for personnel and readiness, Matthew Donovan, to begin working with the staff of the Joint Chiefs to review how military injuries are tracked and reported - not just TBI cases but battlefield injuries of all kinds, Hoffman told reporters.
"The goal is to be as transparent, accurate, and to provide the American people and our service members with the best information about the tremendous sacrifices our warfighters make", Hoffman said.