SEOUL - South Korea's anti-trust agency said on Wednesday it would impose fines totalling 669 billion won ($581 million) on six companies over their involvement in a liquefied petroleum gas (LPG) price cartel.

The Fair Trade Commission (FTC) said four oil refiners -- SK Energy, GS Caltex, S-Oil and Hyundai Oilbank -- and two LPG companies -- E1 and SK Gas -- engaged in price fixing between 2003 and 2008.

The FTC imposed a fine of 198.7 billion won on SK Gas, 189.4 billion on E1 and 160.2 billion on SK Energy.

GS Caltex, S-Oil and Hyundai Oilbank were fined 55.8 billion won, 38.5 billion and 26.3 billion respectively.

The FTC said it was offering benefits to companies that helped with investigation but did not identify them.

Yonhap News reported that SK Energy, which was the first to report details to the regulator, would be excused paying the fine and SK Gas, the second collaborator in the investigation, would get a 50 percent reduction.

SK Gas and E1 were fined 3.1 billion won over an LPG price cartel in 2002. LPG prices were liberalised in 2001 in South Korea.

The FTC will also separately report the case with E1 to prosecutors, according to Yonhap. ($1=1152.0 Won) (Reporting by Rhee So-eui; Additional reporting by Cho Meeyoung; Editing by David Cowell)