Kookmin Bank, South Korea's largest lender, was suspended from conducting new transactions at its branches in Japan for improper lending.

The four-month ban takes effect on Sept. 4, the Financial Services Agency said in a statement in Tokyo Thursday. The regulator found that some former Tokyo branch managers and employees inappropriately structured loans to stay within approval limits, falsified collateral documents and received cash from borrowers.

The suspension comes a week after the heads of Seoul-based Kookmin and its parent KB Financial Group Inc. were admonished by South Korea's financial watchdog following a probe into the loans and other irregularities. Kookmin is also one of four South Korean banks that were inspected this week by South Korea's antitrust agency after Yonhap News reported authorities suspect they may have colluded to set interest rates.

Former Tokyo branch managers failed to take sufficient steps to prevent transactions with "antisocial forces," a term that the FSA uses to refer to criminal organizations, the regulator said Thursday. The bank was ordered to submit a plan for improving compliance by Sept. 29.