An American service member remains missing after a Black Hawk helicopter crashed off the coast of Yemen on Friday in what U.S. military officials are calling a training accident.

U.S. forces in the region rescued five other troops who were on board the aircraft, U.S. Central Command said in a news release.

A search is ongoing for the missing service member, and the U.S. military said it will investigate the incident.

It’s unclear what caused the Black Hawk to crash.

Officials have not provided information on the identities of those on board or the unit to which the aircraft belongs, saying only that the incident occurred around 7 p.m. local time about 20 miles off Yemen’s southern coast.

The U.S. military maintains a small special operations base near Yemen’s port of Mukalla to facilitate an ongoing campaign targeting al-Qaida loyalists there. The Pentagon has conducted more than 80 airstrikes in Yemen this year, officials say.

Several thousand U.S. sailors and Marines also routinely operate in the region as part of a naval amphibious ready group.

Yemen is in the midst of major humanitarian crisis fueled by civil war.

While the U.S. has remained focused on striking al-Qaida, a coalition led by Saudi Arabia – and backed by the Pentagon – has waged a parallel war against Iranian-backed Houthi militants.

Friday’s crash comes just two days after a visit to the Saudi-Yemen border by Gen. Joseph Votel, who oversees the U.S. military throughout the Middle East.

The Washington Post’s Thomas Gibbons-Neff contributed to this report.