The US said on Friday that it would protect its personnel in Syria following air strikes against Iran-backed forces in retaliation for an attack that killed an American contractor and injured five US troops.

A day after a deadly attack on US personnel in Syria, which Washington blamed on an Iranian-made drone, sources say a US base in Syria’s northeast was targeted with a new missile attack. According to US officials, there were no US casualties in the Friday incident.

The latest violence could further aggravate already strained relations between Washington and Tehran amid stalled efforts to revive a nuclear deal and Iran’s military support for Russia’s invasion of Ukraine.

“We’re going to work to protect our people and our facilities as best we can. It’s a dangerous environment,” White House national security spokesman John Kirby said on CNN.

Although U.S. forces stationed in Syria have been targeted by drones before, fatalities are rare.

The Pentagon said the U.S. strikes by F-15 jets on Thursday targeted facilities used by groups affiliated with Iran’s Islamic Revolutionary Guards Corps (IRGC).

The Syrian Observatory for Human Rights, a group that monitors the war in Syria, said the U.S. strikes had killed eight pro-Iranian fighters in Syria.

Reuters was unable to independently confirm the toll.

Iran’s state Press TV, which said no Iranian had been killed in the attack, quoted local sources as denying the target was an Iran-aligned military post, but that a rural development center and a grain center near a military airport had been hit.

“We will always take all necessary measures to defend our people and will always respond at a time and place of our choosing,” U.S. Army General Erik Kurilla, who oversees American troops in the Middle East, said in a statement.

The U.S. strikes were in response to an attack earlier on Thursday by a drone against U.S. personnel at a coalition base near Hasakah in northeast Syria.

Three service members and a contractor required medical evacuation to Iraq, where the U.S.-led coalition battling the remnants of Islamic State has medical facilities, the Pentagon said.

The other two wounded American troops were treated at the base, it said.

On Friday, the Pentagon said the injured personnel were in stable condition.