Banking giant HSBC said on Friday some of its websites had been hit by a "large scale" cyber attack that disrupted online services, but it assured customers that their data were not compromised.
The bank said in a statement that HSBC servers came under a "large scale denial of service attack" on Thursday.
It said a number of sites were affected around the world but did not give an exact number or say where they were.
"This denial of service attack did not affect any customer data, but did prevent customers using HSBC online services, including Internet banking," the bank said.
"We are taking appropriate action, working hard to restore service," the bank added. It said some of the sites are back up and running.
A denial of service attack typically involves sites being saturated with requests.
The London-headquartered, Asia-focused lender said it is working with authorities to investigate the incident. It gave no indication of who it believed might be behind the attack.