The Manila-bound plane, which was originally scheduled to take off at 7:45 a.m. (2245 GMT on Thursday), departed Incheon at around 11 a.m. (0200 GMT), after security checks identified no immediate terror risk, according to the police.

The 9 millimetre bullets were found by a passenger shortly before the jet was to take off with 218 passengers and 12 crew members on board, leading to the plane to return to the terminal.

"We are looking into how the bullets got into the plane," a police officer said.

A Korean Air official said the airline was monitoring the results of the police investigation.

South Korea has strict gun laws with illegal possession of a firearm subject to a prison sentence of up to 15 years and a fine of up to 100 million won ($75,300).

($1 = 1,328.5800 won)

(Reporting by Soo-hyang Choi; Editing by Jamie Freed)