U.S. President Donald Trump speaks at a signing ceremony for the National Defense Authorization Act for Fiscal Year 2020 at Joint Base Andrews, Maryland Dec. 20, 2019. Photo: Xinhua

Thirty-four US troops had been diagnosed with concussions and traumatic brain injury (TBI) as a result of the Iranian missile attack on US military base in Iraq earlier this month, the Pentagon said on Friday.

"Eight service members who were previously transported to Germany have been transported to the United States, they would continue to receive treatment in the United States either at Walter Reed or their home bases," Pentagon spokesman Jonathan Hoffman said at a press briefing.

Hoffman said that nine service members were still undergoing treatment in Germany, and the rest of the 17 injured troops have already returned to duty in Iraq.

US military said last week that 11 service members were treated for concussion symptoms from the blast of Iranian missiles.

Hoffman noted that the symptoms "are late developing and manifested over a period of time."

Seeking revenge for the killing of Iranian senior commander Qassem Soleimani by a US drone strike on Jan. 3 in Iraq, Iran retaliated on Jan. 8 by launching ballistic missiles on military bases housing US troops in Iraq's western province of Anbar and near the city of Erbil, capital of the semi-autonomous region of Kurdistan.

U.S military initially said that no casualty was reported from the Iranian attack. President Donald Trump then downplayed the seriousness of those injures.

"I heard that they had headaches and a couple of other things, but I would say and I can report that it's not very serious," Trump told reporters on Wednesday at a press conference in Davos, Switzerland.

More than 5,000 US troops are deployed in Iraq to support the Iraqi forces in the battle against Islamic State militants.