MPA began investigations after it was notified on March 14 that a number of ships had been supplied with high-sulphur fuel oil (HSFO) containing high concentration levels of chlorinated organic compounds (COC)in Singapore.

MPA said in April that the tainted oil was supplied by Glencore Singapore Pte Ltd.

In further investigations to trace the oil's origin, MPA said the tanker that loaded the oil from Khor Fakkan had discharged the cargo into floating storage facilities in Tanjong Pelepas, Malaysia for further blending, before being delivered to storage facilities in Singapore.

Glencore had purchased the fuel through Straits Pinnacle Pte Ltd, which had contracted its supply from Unicious Energy Pte Ltd, MPA, which oversees the world's largest bunkering port, said in a statement late on Thursday.

Part of the blended HSFO was also sold by Glencore to PetroChina International (Singapore) Pte Ltd.

Glencore declined to comment. The Khor Fakkan port, PetroChina, Singapore-registered oil trading companies Straits Pinnacle and Unicious did not immediately respond to requests for comment.

MPA said that it had found fuel on board the tanker to contain high concentrations of COC, of up to 21,000 parts per million (ppm).

MPA did not name the tanker involved and indicate where the oil was originally produced. Ship-to-ship oil transfers often take place at Khor Fakkan where the oil's origin is unknown.

Glencore and PetroChina had supplied the affected fuel to about 200 ships, and 80 of these ships had reported various issues with their fuel pumps and engines, MPA said in April.

MPA said Glencore and PetroChina had tested the fuel in line with international standards, but did not detect the contamination as current international standards do not require tests for COC.

COC will be included in the list of chemicals to be tested in bunker quality fuel assurance tests, MPA said.

(Reporting by Isabel Kua; Editing by Florence Tan and Elaine Hardcastle)

By Isabel Kua