A vessel was attacked in the Gulf of Aden on Sunday, offshore of Yemen's port of Mukalla, the United Kingdom Maritime Trade Operations (UKMTO) said, without giving further details.
"Vessels transiting the area are advised to exercise extreme caution," UKMTO said in a warning notice on its website. The attack happened at 1230 GMT, it said.
Maritime security firm Dryad Global said it was the ninth reported incident in the Gulf of Aden this year.