Aberdeen Asset Management's head of Asian operations warned that Chinese money was moving "a bit like a casino" in domestic stock markets, while BlackRock called on China to reform its capital markets further to avert boom and bust scenarios.
China stocks have jumped nearly 80 percent since November to trade near seven-year highs. Shanghai Stock Exchange trade surpassed 1 trillion yuan ($161 billion) for the first time, making the bourse the world's biggest in terms of turnover ahead of the New York Stock Exchange.
Recent strong gains have come despite stretched valuations, prompting a warning from the official Xinhua news agency against "irrational exuberance".
Aberdeen's managing director for Asia, Hugh Young, said he was worried about some speculative activity in the Chinese stock market.
"Chinese money is moving a bit like a casino and it's moving offshore. Within the domestic Chinese market, the levels of turnover are tremendous," he told Reuters on the sidelines of a Credit Suisse conference in Singapore.
His comments came after Larry Fink, CEO of BlackRock, the world's biggest money manager, said his firm believes China needed more robust capital markets.
"And by having a more robust capital market, it will mean we'll have less boom bust. Right now, we are experiencing typical boom bust. Let's hope it doesn't end poorly."
Fink added China should expand its capital markets, and needs more IPOs and better underwriting standards.
But Aberdeen's Young said he was not worried about recent stock gains in Hong Kong because the market had not run up like China. Hong Kong's benchmark index has gained nearly 12 percent so far in April, but is up about 18 percent for the year to date.
Aberdeen's China exposure is largely through Hong Kong- listed firms as not many mainland companies meet its quality criteria. Key stocks in the Aberdeen Global - Asia Pacific Equity Fund include AIA Group, Standard Chartered and HSBC Holdings.
Not all speakers at the conference were downbeat in tone about China's stock markets.
Robert Parker, a senior advisor for private banking and wealth management at Credit Suisse, told Reuters he did not think China equity markets were in a bubble.
"Chinese equity markets, nine months ago, were exceptionally cheap, with the price-earnings ratio of less than 10. We have now moved up. We are no longer in super cheap territory," he said.
