Two tankers suspected of illegal ship-to-ship oil transfers were detained and more than 512 million ringgit (US$129.9 million) worth of crude oil seized 24 nautical miles west of Muka Head, Penang last week, the Malaysian Maritime Enforcement Agency said on Saturday, Reuters reported. 

The agency did not specify the origin of the oil being transferred.

The waters off Malaysia are known as a regular site for illegal ship-to-ship transfers, where oil is shifted between tankers at sea to obscure its origin. Malaysian authorities said in July last year they would more tightly enforce rules around the practice, according to Reuters.