A hacker claiming to be behind a cyber attack on Banque Cantonale de Geneve on Friday divulged confidential client information after the Swiss bank failed to meet demands for payment.

In the latest case of a breach of customer information at a financial firm, an anonymous person or group using the Twitter moniker Rex Mundi said it had hacked the Genevan cantonal (state) bank's servers and downloaded more than 30,000 emails by Swiss and foreign clients.

Hours after the hacker's 1700 GMT ultimatum expired, the bank issued a statement saying that the intercepted material had been published, but added that it represented "no particular financial risk for clients or the bank".

"At first analysis, this information is hardly critical, is obsolete or corresponds to foreseen contents about which it has already informed a significant number of clients concerned."

BCGE spokeswoman Helene De Vos Vuadens said that at this stage it appeared that all of the 30,000 mails which the hackers claimed to have intercepted had been published, including some affecting foreign clients.

She said that all the information was from clients' inquiries over the Internet and did not involve their accounts, which require several passwords or codes to access.

The hacker had earlier posted names, addresses and messages to the bank from two people it said were BCGE clients, and said the remainder of the data it had stolen would be make public later on Friday if it was not paid 10,000 euros ($11,779).

"We chose not to give in to blackmail and chose instead the path of transparency," the bank's spokeswoman said.

The hacker played on Swiss banks' reputation for helping clients conceal information from tax authorities.

"We would like to wish a merry tax audit to all the non-Swiss account holders listed in the BCGE files," the Rex Mundi account tweeted on Friday, one of 61 messages that range from detailing demands, to criticizing the bank for lax technology security, and taunting clients who may be at risk.

BCGE is one of a host of Swiss banks to come forward under a government-brokered scheme for banks to pay fines for helping wealthy Americans avoid tax. [ID:nL6N0JZ3R4]

"As for fiscal risks, we asked all of our clients to regularise their (tax) situation. It is up to each of our clients to fulfill legal requirements in conformity with the legislation in force," the BCGE spokeswoman said on Friday.

© Thomson Reuters 2015