The US military says 34 service members have been diagnosed with traumatic brain injury following missile strikes by Iran on a base in Iraq earlier this month, a number higher than previously announced.

Key points: The service members had symptoms like headaches, dizziness, sensitivity to light and nausea

The service members had symptoms like headaches, dizziness, sensitivity to light and nausea Mr Trump appeared to play down the injuries last week

Mr Trump appeared to play down the injuries last week Thousands of Iraqis protested in Baghdad demanding the removal of US troops

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

Iran's attack on the Al Asad and Erbil airbases was a 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 Revolutionary Guard.

Mr Trump and the US military initially said there were no casualties after the strikes.

Last week, the US military said 11 US troops had been treated for concussion symptoms after an attack on the Al Asad airbase in western Iraq, and this week said additional troops had been moved out of Iraq for potential injuries.

A Pentagon spokesman told reporters that 17 service members diagnosed had already returned to duty in Iraq.

Eight service members who had been previously transported to Germany had been moved to the US and would receive treatment at either Walter Reed military hospital or their home bases.

The Pentagon said the service members were being treated on an outpatient basis and were transported back to the US to be closer to their pre-deployment bases.

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

Donald Trump played down injuries

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 5 seconds 2 m 5 s Revolutionary Guard missiles hitting the Al Asad airbase.

The military had seen symptoms like headaches, dizziness, sensitivity to light and nausea, the Pentagon added.

On Wednesday (local time), Mr 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.

While the US military has to immediately report incidents threatening life, limb or eyesight, it does not have an urgent requirement to do so with suspected traumatic brain injury, or TBI, which can take time to manifest and diagnose.

US Defence Secretary Mark Esper had directed the Pentagon to review the process for tracking and reporting injuries.

"The goal is to be as transparent, accurate and to provide the American people and our service members with the best information," Jonathan Hoffman, a Pentagon spokesperson, said.

Various health and medial groups for years have been trying to raise awareness about the seriousness of brain injuries, including concussions.

According to Pentagon data, about 408,000 service members have been diagnosed with traumatic brain injury since 2000.

Protests demand expulsion of US troops

Protesters demanded American troops leave the country. ( AP )

Tens of thousands of Iraqis rallied on Friday (local time) in central Baghdad calling for the expulsion of US troops, but the protest mostly dissipated after a few hours despite a cleric's call for a "million-strong" turnout.

Anti-American marchers wore Iraqi flags and symbolic white robes while holding signs reading "No America, no Israel, no colonialists".

"We want them all out — America, Israel, and the corrupt politicians in government," Raed Abu Zahra, a health worker from southern Samawa, said.

The protests have shattered nearly two years of relative calm following the 2017 defeat of Islamic State and threaten to send the country back into major civil strife.

Unrest erupted in October with protests against a corrupt ruling elite, including Iran-backed politicians, that have met deadly force from government security forces and pro-Iran paramilitaries that dominate the state.

Washington's killing this month of Iranian military general Qassem Soleimani added a new dimension to the crisis.

Iraqi President Barham Salih posted a photo of Friday's march on Twitter and wrote that Iraqis deserved a "fully sovereign state that serves its people".

Security forces used live ammunition and tear gas to disperse a separate anti-government protest that broke out in the early evening in Baghdad's Mohammed al-Qassim highway, security and medical sources said.

Two protesters were killed and 25 wounded following violent clashes.

Erbil airbase and Al Asad airbase were hit in the attacks. ( ABC News )

Reuters/ABC