TOKYO (Kyodo) -- The head of MUFG Bank apologized on Monday over the alleged theft of more than 1 billion yen ($6.5 million) by a former employee from customers' safe deposit boxes, vowing to boost compliance at one of the biggest megabanks in Japan.

"The incident shook the foundations of the banking business, which is built based on trust," MUFG Bank President Junichi Hanzawa said at a press conference. "We will do our utmost to dispel the concerns of customers who use our safe deposit boxes."

The apology came after the revelation last month that the former employee, who was in charge of managing the spare keys for safe deposit boxes at some of MUFG Bank's branches in Tokyo, allegedly stole assets worth between 1 billion and 2 billion yen.

The bank said the total amount of damage could expand as it is still reviewing complaints from customers. The former employee, a woman in her 40s, said she had no accomplices in the alleged theft, according to the bank.

The incident came to light on Oct. 31 after the bank received an inquiry from a client, MUFG Bank said, adding that it fired the employee, on Nov. 14.

The bank has unveiled a series of measures to strengthen compliance, including storing the spare keys for safe deposit boxes at its headquarters rather than at branches.