South Korea's antitrust agency is investigating the country's four largest commercial banks on suspicion of collusion on interest rates, news agency Yonhap reported on Wednesday.

Yonhap said investigators from the Fair Trade Commission visited Kookmin Bank, Woori Bank, Shinhan Bank and Hana Bank on Tuesday and Wednesday.

Earlier, people with knowledge of the matter had told Reuters the regulator was investigating at least four banks for possible interest rate collusion.

A FTC spokesman declined comment. Kookmin, Woori and Hana declined to comment, while a spokesman from Shinhan Bank could not be immediately reached.