"Authorities are investigating. Vessels are advised to transit with caution and report any suspicious activity to UKMTO," UKMTO said in a statement without providing further details.

British maritime security firm Ambrey said separately that a Bangladesh-flagged bulk carrier was reportedly boarded east of Somalia.

"Merchant vessels are advised to stay well clear of this position as this is assessed to be an ongoing incident with perpetrators attempting to take control of the bulker," Ambrey said in a statement.

--Reuters--