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.

Axar.az reports, citing Reuters, based on initial assessments, the incident appeared to be related to Somali piracy rather than Yemen’s Iran-aligned Houthi militia, one of the 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.

Efforts were under way to assist the Asana tanker and determine the circumstances of what happened, said an official with the European Union’s Aspides naval mission, which is active in the Red Sea and Gulf of Aden region.

A South Korean warship was in the area, the official told Reuters.

“Details regarding the number of assailants, the circumstances of the boarding, and the status of the vessel and crew remain unclear,” British maritime risk management group Vanguard said.

The vessel issued a distress call at around 0620 GMT on Friday and did not have an armed security team when the incident occurred, British maritime security group Ambrey said, adding that the assailants were suspected to be part of a pirate action group.

The vessel’s operator was listed in shipping databases as Marshall Islands based Exon Energy, which could not be reached for comment.

Iran has asked Yemen’s Houthis to stand ready to close the Red Sea oil route if the United States strikes Iranian power infrastructure, sources told Reuters on Thursday, posing a potent new threat to global energy supplies.

A vessel was boarded by unauthorized 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 earlier on Friday.