LONDON, July 17 (Reuters) - Armed assailants are believed to have boarded the chemical tanker Asana off the southern coast of Yemen in the Gulf of Aden on Friday and are in control of the vessel, maritime security sources said.

The small tanker, which had no confirmed flag, had listed the Somali port of Bosaso as its next destination, ship tracking data showed.

A vessel was boarded by unauthorised personnel while transiting east in the Gulf of Aden, 65 nautical miles south of Yemen's Al Mukalla port, the British navy agency UKMTO said on Friday.