The Pentagon said on Friday that 34 United States service members had been diagnosed with traumatic brain injury following missile attacks by Iran on a base in Iraq earlier this month.

Trump and other top officials initially said Iran's attack had not killed or injured any US service members.

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Last week the US military said 11 US troops had been treated for concussion symptoms after the attack on the Ain al-Assad airbase in western Iraq and this week said additional troops had been moved out of Iraq for potential injuries.

The January 8 attack was retaliation for a US drone strike in Baghdad on January 3 that killed Qassem Soleimani, the commander of the elite Quds Force of Iran's Islamic Revolutionary Guard Corps (IRGC).

Pentagon spokesman Jonathan Hoffman told reporters on Friday that eight service members who had been previously transported to Germany had been moved to the US. Hoffman said the service members were transported earlier on Friday and would receive treatment at either Walter Reed military hospital or their home bases.

Nine service members remain in Germany and are undergoing evaluations and treatment.

On Wednesday, Trump appeared to play down the injuries, saying he "heard that they had headaches and a couple of other things".

Pentagon officials have said there had been no effort to minimise or delay information on concussive injuries, but its handling of the injuries following Tehran's attack has renewed questions over the US military's policy regarding how it deals with suspected brain injuries.