The strike was in response to a February missile attack on a facility in Iraq where the U.S. military is stationed
The U.S. Defense Department estimates that a U.S. military strike on a facility in Syria used by Iranian-backed militias killed one soldier and wounded two others.
Nine buildings were destroyed in the attack.
Pentagon spokesman John Kirby released the casualty figures at a briefing about four days after the Joe Biden administration carried out the attack on a border post in eastern Syria.
The U.S. military said the border post was being used by Iranian-linked Iraqi militias who are pressuring U.S. forces in Iraq.
The U.S. military strike was the first lethal operation ordered by the Biden administration against Iranian-controlled forces in the Middle East.
It was in response to a Feb. 15 missile attack on a facility in northern Iraq where the U.S. military is stationed that killed a civilian contractor and injured several people, including a U.S. serviceman.
U.S. officials said the border crossing was used by Iranian-backed militias in Iraq who U.S., Kurdish and Iraqi authorities have determined were responsible for the January attack.