UK maritime security reporting body UKMTO has revealed a cargo vessel was approached by a large number of fast-moving skiffs in the Bab el Mandeb strait on Thursday.
The "suspicious approach" featured four or five people in each boat.
The on-board security team fired warning shots to deter boarding.
The crew and the vessel were later reported safe.
"Vessels transiting the area are advised to exercise extreme caution," UKMTO said.
