A US Black Hawk helicopter crashed off the southern coast of Yemen on Friday during a training mission, leaving one service member missing, the US military said.

US Central Command said in a statement that five other service members aboard the aircraft had been rescued after the crash, which took place Friday evening about 32 km off the southern coast of Yemen.

A US official told Reuters that the cause of the crash was under investigation.

“When the incident took place the helicopter was not very high above the water,” CENTCOM spokesman Colonel John Thomas said.

Asked if the crash involved another special forces raid, Central Command told The Associated Press that “this was a routine training event specifically for US military personnel.”

“Training events such as this are routinely held by US forces within a theater of operations in order to maintain their proficiency within the operating environment,” CENTCOM told the AP in a statement. “Commanders deemed this location appropriate and safe for a routine training event, considering both the operational environment and weather conditions at the time.”

The United States has been carrying out air strikes against al-Qaeda in Yemen, with at least 80 launched since the end of February.

A small number of ground raids using US Special Operations forces have also taken place, including one in January which resulted in the death of a US Navy Seal.