The missing personnel have not been identified, and it was not immediately clear what vessel they were attempting to board, or why. U.S. forces routinely partner with other nations as part of a counter-piracy task that operates in the Gulf of Aden. Those missions sometimes include boarding vessels to ensure they have proper credentials or are not transporting illicit material.

One U.S. official with knowledge of the incident said it was unrelated to the recent U.S.-led strikes in nearby Yemen and the broader international mission to protect commercial vessels from militant attacks originating there. It was also unrelated to the Iranian seizure of a Marshall Islands-flagged tanker, the official said.

U.S. Central Command, which coordinates military activities in the region, said the missing service members were Navy sailors and declined to provide additional information “until the personnel recovery operation is complete.”

Special Operations forces in the region have faced difficult counterterror missions and other difficult operations. In November, five crew members of an elite aviation unit were killed during a refueling accident off the coast of Cyprus.

ABC News was first to report on the missing SEALs.

This is a developing story and will be updated.