The US conducted an airstrike in Syria against what it said were Iranian-affiliated facilities after a suspected Iranian drone on Thursday struck a facility housing US personnel in the country, killing an American contractor and wounding five US service members.

The contractor was an American citizen, a spokesman for US Central Command confirmed, and an additional US contractor was also wounded in the strike. An official familiar with the matter told CNN that the injured service members are all in stable condition.

“The intelligence community assess the UAV (unmanned aerial vehicle) to be of Iranian origin,” the Pentagon said.

In response to the strike, President Joe Biden authorized a precision airstrike “in eastern Syria against facilities used by groups affiliated with Iran’s Islamic Revolutionary Guards Corps (IRGC),” Secretary of Defense Lloyd Austin said in the statement.

The US, according to the Pentagon statement, “took proportionate and deliberate action intended to limit the risk of escalation and minimize casualties.”

“As President Biden has made clear, we will take all necessary measures to defend our people and will always respond at a time and place of our choosing,” Austin said. “No group will strike our troops with impunity.”

The strikes are likely to increase tensions with Iran, with which the proxy groups are aligned, though Tehran isn’t always involved in directing attacks that they conduct. The US has already sanctioned Tehran for providing attack drones to Russia to use in the war in Ukraine. And on Thursday, Chairman of the Joint Chiefs of Staff Gen. Mark Milley reiterated US concerns that Iran has the potential to produce enough fissile material for a nuclear weapon in less than two weeks and manufacture one within months.

US target was supporting Iranian groups, official says

The drone intentionally crashed into its target, the official said. The infrastructure that was targeted in the US response was not directly related to the suspected Iranian drone itself, the official said, but was instead targeted by the US because it was known to be supporting Iranian proxy groups in the country with munitions and intelligence.

The number of casualties from the US airstrike is still being determined, the official said.

The commander of US Central Command, Gen. Erik Kurilla, said the US could carry out additional strikes if there were more attacks. “We are postured for scalable options in the face of any additional Iranian attacks,” Kurilla said in a statement Thursday evening.

The US maintains approximately 900 troops in Syria.

Kurilla said earlier Thursday that Iranian proxies had carried out drone attacks or rocket attacks against US forces in the Middle East 78 times since the beginning of 2021, an average of nearly one attack every 10 days.

“What Iran does to hide its hand is they use Iranian proxies,” Kurilla told a House Armed Services Committee hearing earlier in the day. “That’s either UAVs or rockets to be able to attack our forces in either Iraq or Syria.”

Asked if such attacks were considered an act of war, Kurilla said, “They are being done by the Iranian proxies is what I would tell you.”

Latest US strike against Iranian assets in region

The Biden administration has carried out airstrikes against militias affiliated with Iran on multiple occasions following previous attacks on US facilities in the region.

In February 2021, Biden’s first known military action was to carry out strikes against Iranian-backed militias after rocket attacks on US troops in Iraq. And in August, the US struck a group of bunkers used for ammunition storage and logistics support by Iranian proxies in Syria, after rockets landed near another US facility.

Milley visited US troops in Syria earlier this month, marking the first time he has visited as the top US general. Milley visited troops in northeast Syria who are there as part of the ongoing campaign to defeat ISIS, a mission the US carries out with its partners in the Kurdish-led Syrian Democratic Forces.

But Milley’s visit also focused on the safety of US troops, his spokesman had said, and he inspected for protection measures in Syria.

Two weeks before Milley’s visit, US and coalition forces at Green Village in Syria came under rocket attack. No US or coalition troops were injured in that attack, but it underscored the threat emanating from adversaries in the region, often in the form of Iranian-backed proxies or militias.

Just two days before the rocket attack, four US troops and one working dog were injured in a helicopter raid against a senior ISIS leader in northeast Syria.

This story has been updated with additional details.

CNN’s Haley Britzky contributed to this report.