By Xinghui Kok and Archishma Iyer

The Monetary Authority of Singapore (MAS) on Thursday told DBS Group Holdings' unit DBS Bank and Citibank Singapore to conduct a thorough investigation after they failed to recover their systems fully within a required timeframe during an unscheduled outage on Oct. 14.

The central bank said unscheduled downtime for a critical system that affects a bank's operations or service to customers must not exceed four hours within any 12-month period.

On Saturday Oct. 14, DBS's and Citibank's online banking and payment services went offline from around 3 p.m. Singapore time with services only fully resuming on Sunday morning, Singapore broadcaster CNA reported. Both used a data centre operated by Equinix.

Equinix said on Saturday it was a technical issue and that it was conducting a thorough investigation into the event.

MAS said DBS and Citibank had activated their back-up data centres when their primary data centres failed to perform normally, but the banks were not able to fully recover their systems within the required timeframe.

"MAS has instructed both banks to conduct a thorough investigation on why they were not able to do so, and will take appropriate supervisory actions after gathering the necessary facts," the central bank said in a statement.

"No IT system is infallible. Banks and customers should have contingency measures in the event of service disruptions caused by IT outages."

In May, the central bank had imposed an additional capital requirement on DBS Bank after a widespread unavailability of the bank's digital banking services in March.

(Reporting by Archishma Iyer in Bengaluru and Xinghui Kok in Singapore; Editing by Shinjini Ganguli and Susan Fenton)