Hong Kong Exchanges and Clearing initially claimed a "technical problem" with its HKExnews Web site meant investors could not access price sensitive company announcements.

Trading was halted in eight companies as the operator adopted a half day suspension policy for issuers which announced price sensitive information to give investors a chance to locate announcements on their Web sites.

Speaking later in the day, Charles Li, CEO, HKEx, told reporters that "our current assessment that this is a result of a malicious attack by outside hacking. We are working on it, hopefully we can be back online as soon as we can."

Li was speaking after the exchange posted a second quarter net profit of HK$1.35 billion, up 19.5% on the same period the previous year.

The attack on HKEx follows a wave of similar cyber-espionage at other exchanges, including Nasdaq OMX. Earlier this year, stock exchanges in the UK and US called in security services to help fend off malicious assaults, which are believed to be linked to state-sponsored infiltrators.