Six U.S. troops have Surfwinbeen diagnosed with traumatic brain injuries (TBI) following last week's Iranian-backed attacks in Syria that killed a U.S. contractor, the Pentagon said Thursday.

The six TBI diagnoses bring the number of Americans who sustained injuries in the attacks to 13. 

"In addition to the seven injured service members that I highlighted, there were an additional six U.S. service members that have subsequently been diagnosed with traumatic brain injury as a result of the-Iranian backed attacks," Pentagon Press Secretary Brig. Gen. Patrick Ryder said Thursday. 

Four of the service members who suffered TBIs were involved in the strike on March 23 while two others were injured in the attack on March 24, according to Ryder. The TBIs were found during routine screening for personnel who were in the vicinity of an explosion, Ryder said.

It was possible that, as those screenings continued, more TBIs could be diagnosed, Ryder noted. 

TBIs can be similar to concussions and present symptoms like headaches, dizziness, irritability, fatigue or poor concentration. 

The seven Americans who sustained wounds were in stable condition, although one was wounded seriously enough to require evacuation to Landstuhl, according to Ryder. 

The series of attacks began last week when a drone targeted US forces in northeast Syria, killing a U.S. contractor and wounding six other Americans. The U.S. retaliated with F-15E fighter strikes against sites used by groups backed by the Islamic Revolutionary Guard Corps. 

Defense Secretary Lloyd Austin told Congress Tuesday the sites "included a headquarters element building and also storage sites, where key munitions were being stored and other capabilities were being developed."

According to Ryder, the U.S. retaliatory strikes killed eight militants. 

After the retaliatory strikes, the Iranian-backed militias carried out three more attacks on bases in Syria. In one of them, another U.S. service member was injured but is in stable condition. 

Austin told Congress on Thursday that there have been about 83 attacks on U.S. forces in Syria by Iranian-backed proxies in the last several years. 

— Paulina Smolinksy contributed reporting.

In:
• Iran
• Syria

Eleanor Watson

CBS News reporter covering the Pentagon.

Twitter