The United States military carried out airstrikes against an eastern Syrian facility used by Iranian-backed militias late on Wednesday.
The Pentagon said it was in retaliation for a growing number of attacks on bases housing US troops in the region for the past several weeks.
"The President has no higher priority than the safety of US personnel, and he directed today's action to make clear that the United States will defend itself, its personnel, and its interests," Defense Secretary Lloyd Austin said in a statement.
It marks the second time in recent weeks that the US has taken such action.
Iran's Revolutionary Guard targeted
The London-based Syrian Observatory for Human Rights told the French news agency AFP that nine people "working for Tehran-backed groups" were killed in the attack.
Austin said two F-15 fighter jets carried out the strike against a weapons storage facility linked to Iran's Revolutionary Guard.
Since the start of October, US and coalition troops have been attacked at least 40 times in Iraq and Syria by Iran-backed forces.
Austin warned the US was "fully prepared to take further necessary measures to protect our people and our facilities."
The US has 900 troops in Syria and 2,500 in Iraq to assist local forces in preventing an resurgence of the so-called "Islamic State" group.
The strikes come amid heightened tensions in the region due to the war between Israel and Hamas.