More than 100 American service members have traumatic brain injuries from Iranian airstrikes on Al Asad Air Base in Iraq in January, the defence department said, a number that was more than 50 per cent higher than previously disclosed.

Of the 109 troops who have been diagnosed with brain injuries, 76 had returned to duty, officials said on Monday.

“We are grateful to the efforts of our medical professionals who have worked diligently to ensure the appropriate level of care for our service members, which has enabled nearly 70 per cent of those diagnosed to return to duty,” said Alyssa Farah, the Pentagon press secretary.

The latest tally, which has steadily grown since the January 8 strike, drew a sharp contrast with the assertion by the Trump administration in the hours after the attack that no Americans were hurt. The number also underscored the unseen effects of traumatic brain injuries, which sometimes do not manifest symptoms for days or weeks but can have long-term physical or mental effects.

And as the injury toll has mounted, veterans groups and others have levied criticism at the White House, in part because, in January, President Trump dismissed the injuries as “not very serious”.

“I heard that they had headaches and a couple of other things,” Trump said at a news conference on January 22 in Davos, Switzerland.

“I don’t consider them very serious injuries relative to other injuries I have seen.”

At least a dozen missiles were fired during the attack, which was a retaliation for the killing of a top Iranian general, Qassem Soleimani, by an American drone strike in Baghdad on January 3. The Trump administration at first said there were no injuries, but a week later said several service members were evaluated for possible concussions.

Then, days after Trump’s statements in Davos, the defence department said that 34 people had suffered brain injuries. The number was later increased to 50 and then to 64, with military officials saying that the symptoms of traumatic brain injuries could take weeks to appear.

The repeated revisions have drawn outrage from some veterans and senators.