Nearly three dozen US troops suffered traumatic brain injuries or concussion in this month's Iranian airstrike on a military base in Iraq, despite President Trump's claim that no Americans had been hurt.
Mr Trump had initially said that no US personnel were injured in the strike on the Ain al-Asad base in western Iraq on January 8 although authorities later reported that 11 troops were injured.
Democrats have accused the president of lying and Joe Biden called Mr Trump'a actions "disgusting".
"Thirty-four total members have been diagnosed with concussions and TBI (traumatic brain injury)," Pentagon spokesman Jonathan Hoffman said on Friday.
Mr Hoffman said that 17 of the victims had been initially transferred to Germany to receive treatment, eight of whom arrived back in the US on Friday.
"They will continue to receive treatment in the United States, either at Walter Reed (military hospital near Washington) or at their home bases," he said, adding that the nine other victims who were flown to Germany "are still undergoing evaluation and treatment there".
Seventeen other troops who were treated in the region have returned to duty in Iraq.
The airbase - one of the largest in Iraq, with 1,500 US troops making up the bulk of a coalition presence directly adjacent to thousands of Iraqi forces - was targeted in retaliation for the US killing of top Iranian general Qassim Soleimani in a January 3 drone strike in Baghdad.
Democratic lawmakers seized on the announcement to accuse Mr Trump of lying about the strike.
"Though he downplays these injuries as 'headaches,' roughly half of all TBIs require surgery and can lead to lifelong disabilities," said Debbie Wasserman Schultz, a Florida congresswoman.
Jack Reed, a senator and the Ranking Member of the Armed Services Committee, called TBI "a serious matter."
"It is not a 'headache,' and it's plain wrong for President Trump to diminish their wounds. ... He owes them an apology," Mr Reed said.
At a rally in New Hampshire, Joe Biden, a frontrunner to challenge Trump in this year's election, said the president "brushed off" the troops' injuries.
"I find it, quite frankly, disgusting," Mr Biden, a former vice-president, said.
