The Blackhawk helicopter crash occurred on San Clemente Island, about 70 miles (around 112 km) west of San Diego, a US Defense Department official with knowledge of the incident told a local news outlet, the San Diego Union-Tribune.

The soldiers involved in the incident were assigned to an element of the US Army's Special Operations Command and were conducting routine training, the local daily newspaper in San Diego reported, citing a statement from the US Army.

"We are still in the process of making next of kin notifications and will not release any additional information until that process is complete," J. Elise Van Pool, a US Army spokeswoman was quoted as saying by the paper.

Officials said that an investigation is underway to determine the cause of the accident.

It is the second fatal accident during military training exercises in the area in a month. On July 30, nine US troops were killed when their amphibious assault vehicle sank near San Clemente Island.