The United States treated 11 of its troops for symptoms of concussion after an Iranian missile attack targeted an Iraqi base where US forces were stationed, the US military has said, after initially saying no service members were hurt.

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 Revolutionary Guard.

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US President Donald Trump and the US military had said there were no casualties after the attack on the Ain al-Asad airbase in western Iraq and a facility in its northern Kurdish region.

At the time of the attack, most of the 1,500 US soldiers at the base had been tucked away in bunkers, after advance warning from superiors.

"While no US service members were killed in the January 8 Iranian attack on al-Asad airbase, several were treated for concussion symptoms from the blast and are still being assessed," Captain Bill Urban, spokesman for US Central Command, said in a statement on Thursday.

As a measure of caution, some service members were taken to US facilities in Germany or Kuwait for "follow-on screening," he added.

"When deemed fit for duty, the service members are expected to return to Iraq."

Reporting from Washington, DC, Al Jazeera's Gabriel Elizondo said many of the troops were screened for concussions, which the US authorities say is common.

"The blast from missiles can cause buildings to shake and rattle and can cause potentially, concussions even for people who are outside of the immediate impact zone," Elizondo said.