A U.S. military helicopter shot down by enemy fire crashed in eastern Afghanistan late Friday, killing all 31 U.S. special operations troops and seven Afghan commandos on board, The Associated Press reports.

In a statement, President Barack Obama mourned the American troops who perished, saying the catastrophe serves as a reminder of the “extraordinary sacrifices” being made by U.S. forces and their families. President Obama also expressed regret for the seven Afghans who died alongside U.S. soldiers.

An American official said the plane was apparently shot down after returning from a mission in Sayd Abad district of Wardak, an area known for its strong Taliban presence, the AP reports. The region’s mountainous terrain is hazardous territory for aircraft as insurgents armed with rocket-propelled grenade launchers can fire from unseen vantage points.

The Taliban claimed they shot down the helicopter with rocket fire, making the attack the deadliest single incident for American forces since the war began in 2001, according to the report.

(AP Photo/Anja Niedringhaus, File)