South Korea's financial watchdog and the central bank on Thursday started their joint investigation into the nation's top financial company KB Financial Group Inc., which have been charged with irregularities in the firm's investments and outside directors.

Earlier in the day, the Financial Supervisory Service (FSS) sent 42 inspectors to the group's headquarters and computer system hub, as well as to its main unit Kookmin Bank, while the Bank of Korea (BOK) dispatched nine agents for a four-week audit into the bank's foreign exchange operations and risk management, local media reported.

The investigation is aimed at checking whether the group's investment on Bank Center Credit in Kazakhstan, which led into a loss, and issuance of covered bonds worth 1 billion U.S. dollars in May were in breach of financial market regulations. The financial conglomerate has been spotlighted for market allegations that the financial authorities stepped in the process of Kookmin Bank chief Kang Chung-won's resignation on Dec. 31 from the chairman of the parent company.

Amid market speculation that the government had an influence, Kang only cited the interest of the group as he stepped down.