SEOUL -- A spare power bank is a possible cause of a fire that engulfed an Air Busan 298690.KS plane in January, South Korea's transport ministry said on Friday, citing interim investigation results.

Scorch marks on the debris of a power bank found where the fire was first detected indicate the blaze may have started because insulation inside the battery had broken down, the statement said.

Investigators cannot yet state what may have caused the breakdown, however, according to the statement.

Lithium batteries in devices such as laptops, mobile phones, electronic cigarettes and power banks can produce smoke, fire or extreme heat when manufacturing faults or damage cause them to short circuit.

No abnormalities in the plane's own electrical systems have been identified, the statement said.

The fire on January 28 was first detected in a luggage bin above row 30 on the left side of the plane about 20 minutes after the delayed flight to Hong Kong from Busan, in South Korea, had been scheduled to depart, investigators have said.

All 170 passengers and six crew were evacuated from the Airbus A321ceo plane, which the fire destroyed.

South Korea's Aviation and Railway Accident Investigation Board is leading the investigation. Friday's investigation update is not a final accident report, which states are obliged to produce within a year of an incident, according to global aviation standards.

Aviation has long recognized lithium batteries as a safety concern, and rules are periodically tightened in response to accidents.

From March 1, South Korea changed rules on carrying batteries onboard flights, including keeping power banks and e-cigarettes with passengers and not in overhead bins, and not charging devices on board.

Last year three incidents every two weeks of overheating lithium batteries on planes were recorded globally by the U.S. Federal Aviation Administration, compared to just under one a week in 2018.