Singapore's central bank said Wednesday that it will consider taking "supervisory action" against DBS Group Holdings Ltd following another day of disruptions to the Singapore bank's online banking services.

DBS shares were down 0.6% in Singapore as of 9:07 a.m. Thursday.

DBS Bank has ascribed a service outage on "access control servers," which the bank says prevented many clients from logging into their accounts.

The local regulator has directed the Singaporean lender to investigate the source of the problem. The breakdown was one of the most severe digital interruptions in a decade for Southeast Asia's largest bank.

"This is a significant disruption, and the Monetary Authority of Singapore (MAS) expects DBS to conduct an exhaustive investigation to ascertain the root causes and implement necessary corrective measures," Marcus Lim, an assistant managing director with the MAS's banking and insurance division, said in a statement.

Earlier in the day on Wednesday, DBS assured customers that their deposits and funds were secure despite the technical difficulties. Customers were outraged by the mistake just hours after DBS declared a patch following the first day of downtime.

Customers were unable to withdraw cash from ATMs for hours because of the disruption, which included its payments app. As a result, the MAS took supervisory action against it.

The bank first provided scant clues about the reason of the incident, stating on its Facebook, Twitter, and website that it was aware of clients suffering "intermittent slowness" when accessing its banking services.

DBS said that the issue had been fixed and services restored. However, clients reported experiencing various types of issues accessing the bank's online services again later in the day, prompting the bank to concede the issue had not been fully resolved.

Financial institutions must ensure that the maximum downtime for each essential system does not exceed four hours in any 12-month period.

DBS has allotted S$230 million ($168 million) in regulatory capital in 2010 following a six-hour outage of its banking services following repairs.

DBS has invested considerably in recent years to automate its core banking operations and establish new technological platforms. These measures have aided in increasing the bank's return on equity and enabled the lender to expand its customer base across all of its markets.

DBS currently has operations in Indonesia, Hong Kong and Inida but its retail and wealth management operations are concentrated in Singapore, where it is the market leader in retail banking.