(Adds regulatory action in U.S., Singapore, details of recent volatility) HONG KONG, Feb 3 (Reuters) - Hong Kong’s markets watchdog warned investors on Wednesday about the risks of trading highly volatile securities as investment discussion forums on social media become more influential. Mass buying by amateur traders over the past two weeks, fuelled by posts on the popular Reddit forum WallStreetBets, has driven wild price gyrations in companies that big U.S. fund managers had bet against, including videogame retailer GameStop and cinema operator AMC Entertainment. This has raised global regulators’ concerns. U.S. Treasury Secretary Janet Yellen is calling a meeting of top U.S. financial regulators this week to discuss the market volatility, a Treasury official told Reuters. The Monetary Authority of Singapore, on Tuesday, also warned investors of risks related to trading in securities incited by online forums and social media chat groups. Hong Kong’s Securities and Futures Commission (SFC) said in a statement it was in close dialogue with local and overseas regulatory counterparts about investor risks in volatile stocks. The statement said investors should be aware that brokers might have rights under the terms and conditions of customer agreements to suspend trading services in some circumstances. It added the SFC would take regulatory action if there was evidence that intermediaries were not acting in the best interests of their clients and integrity of the market.