The Pentagon said on Tuesday that 50 US soldiers were now diagnosed with traumatic brain injury after missile attacks by Iran on a base in Iraq earlier this month, 16 more than the military had previously announced.

President Donald Trump and other top officials initially said Iran's January 8 attack had not killed or injured any US service members.

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

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"As of today, 50 US service members have been diagnosed" with traumatic brain injury, Pentagon spokesman Lieutenant Colonel Thomas Campbell said in a statement about injuries in the attack on the Ain al-Asad airbase in western Iraq.

Symptoms of concussive injuries include headaches, dizziness, sensitivity to light and nausea.

A total of 31 of the 50 were treated in Iraq and returned to duty, including 15 of those diagnosed most recently, Campbell said.

Eighteen of the total have been sent to Germany for further evaluation and treatment, while one was sent to Kuwait and has since returned to duty, he said.

"This is a snapshot in time and numbers can change," Campbell said.

In its previous update on Friday, the Pentagon had put the number of those injured at 34.

Trump earlier said the injuries were "not very serious", prompting criticism from a US war veterans group.

Senator Jack Reed, a Rhode Island Democrat and former Army Ranger, called on Trump to apologise.

"TBI is a serious matter," Reed said in a statement referring to brain injuries.

"It is not a 'headache,' and it's plain wrong for President Trump to diminish their wounds. He may not have meant to disrespect them, but President Trump's comments were an insult to our troops. He owes them an apology."

William Schmitz, national commander of the Veterans of Foreign Wars, also said on Friday the group "expects an apology from the president to our service men and women for his misguided remarks."

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

Iran fired missiles at Ain al-Asad in retaliation for the US killing of a top Revolutionary Guard general, Qassem Soleimani, in an attack at Baghdad airport on January 3.

The missile attacks capped a spiral of violence that had started in late December, and both sides have refrained from further military escalation.