Malaysia has detained two tankers suspected of carrying out an unauthorised ship-to-ship (STS) crude oil transfer off Penang, Malaysia.

The authorities received a report of a suspected STS crude transfer at around 01:00 local time on Thursday, approximately 24 nautical miles west of Muka Head, off Penang, the MMEA said in a social media post on Sunday.

The two vessels had a combined crew of 53 onboard.

According to the MMEA, the seized crude cargo was assessed at more than RM512 million ($128 million), while the two tankers themselves were valued at RM718 million, equivalent to $182 million.

The case remains under investigation, with both vessels and their captains taken into custody for further inquiries.