NEW YORK (AFP) – US banking giant Citigroup had told its clients that about 360,000 credit cards were affected by a computer hacking attack last month -- nearly twice the number previously acknowledged.

Citigroup said late Wednesday that the security breach had compromised a total of 360,083 North America Citi-branded credit cards, after earlier saying the attack impacted around 210,000 credit card customer accounts -- roughly one percent of its US clientele.

"Citi has implemented enhanced procedures to prevent a recurrence of this type of event. We have also notified law enforcement and government officials," the banking group said in a statement to "our customers."

"We continue to monitor customer service and communication channels and take every necessary action to ensure our customers are cared for."

It declined to provide further details of the breach, citing the ongoing investigation and its customers' security.

The company has come under scrutiny from US lawmakers for its handling of the attack. Citi did not inform its clients of the May 10 attack until nearly a month later.

Citi said it has replaced the cards of 217,657 customer accounts, while others were not re-issued cards because their accounts were closed or they were already receiving new cards for other reasons.

Citi is one of the world's largest providers of credit cards, with more than 21 million accounts and is owed more than $77 billion in North America, according to its 2010 annual report.