Hackers targeted one of the world’s largest banks on Friday morning, preventing some of HSBC’s customers in the United Kingdom from being able to access their online accounts.

HSBC issued a statement saying it “successfully defended” against a denial-of-service attack, in which hackers try to prevent people from accessing a given site by overwhelming it with traffic.

The company said the attack targeted its Internet banking system for the United Kingdom, but no transactions were affected. However, some United Kingdom customers who tried to log into their accounts Friday were greeted by a message that said online banking was unavailable.

That message did not appear on the company’s website for online banking in the United States.

HSBC tweeted that its service was recovering, though it was still seeing some denial-of-service attacks some five hours after it initially reported the incident. The bank added it was “working closely with law enforcement authorities to pursue the criminals responsible.”

About 17 million United Kingdom residents are HSBC customers, the bank says. It apologized to all those inconvenienced by the outage, and encouraged them to visit a branch for urgent issues.

It was the second time this month that HSBC customers had an issue with online banking.

The company tweeted that “an internal technical issue” prevented some people from accessing their accounts on Jan. 4 and Jan. 5. In a video tweeted from the company’s account, an HSBC official said that was not caused by a cyber attack and that customers’ data was never at risk.

HSBC has about 6,100 offices in more than 70 countries and territories across the globe, according to its website.