UPDATE 1-HKEX chief says derivatives trading glitch caused by software bug

* CEO Charles Li says trading glitch not due to human error

* Li says bourse website suffered separate DDoS attack

* HKEX says derivatives market operating smoothly (Adds Li quote, details)

HONG KONG, Sept 6 (Reuters) - A derivatives trading glitch at bourse operator Hong Kong Exchanges and Clearing (HKEX) was caused by a software bug and was not due to human error, chief executive Charles Li said on Friday.

Li was speaking a day after HKEX suspended trading in its derivatives market for the afternoon and after-hours sessions “due to prolonged connectivity issues on the Hong Kong Futures Automatic Trading System”.

Derivatives trading was halted on Thursday just as local stocks slipped on profit taking, following an almost 4% rally on Wednesday when the city’s leader formally withdrew a proposed extradition bill that sparked three months of protests.

Meanwhile Li said the HKEX website came under attack on Thursday, Li said, in a Distributed Denial of Service (DDoS) incident that had nothing to do with the suspension in derivatives trading.

DDoS attacks involve sending large numbers of requests in a targeted attack, causing partial or full service interruptions.

“There have been several such attacks this year. In fact, there might still be some this morning. DDoS happen quite frequently for many companies’ websites,” Li said.

“But the website is not our trading system. This only affects the speed with which some clients to read information (on the website).”

HKEX said the derivatives market was operating smoothly on Friday. (Reporting By Alun John, Noah Sin and Donny Kwok, Writing by Anne Marie Roantree; Editing by Sam Holmes)