South Korea's Fair Trade Commission (FTC) said on Thursday that it had decided to impose a total 434.8 billion won ($394.7 million) penalty on four domestic crude oil refiners after finding they colluded in managing gas stations to maintain market share.

The refiners include SK Energy, fully owned by SK Innovation, GS Caltex, S-Oil and Hyundai Oilbank, a statement from the commission said. Its findings on SK Innovation, GS Caltex and Hyundai Oilbank would be forwarded to prosecutors, the statement said.

S-Oil denied the charges in a statement, saying it "never colluded with rivals in managing gas stations" and vowing "various responses including legal action."

Officials from the other refiners were not available for comment. Local media had speculated the penalties could reach 1 trillion won.

 Last month domestic refiners in South Korea, the world's fifth-largest crude oil importer, cut gasoline and diesel prices by 100 won per liter for three months from April 7, facing government pressure to help curb inflation.

Heavily dependent on energy and other commodity imports, South Korea is along with other Asian economies struggling to tame inflation partially fuelled by multi-year high prices for raw materials including oil. South Korea's annual consumer inflation stayed above the central bank's target band of 2-4% for four months in a row, with the inflation expectation rate for the next 12 months climbing to a 22-month high of 4.0% in April.