* CSI300 +0.3 pct; SSEC +0.5 pct; HSI +0.8 pct
* Anti-graft campaign benefits market in the long term - trader
* Cyber-security shares surge ahead of Xi's Washington visit
SHANGHAI, Sept 17 (Reuters) - China stocks extended their rebound on Thursday morning, led by small-caps, as some investors viewed a deepening anti-corruption campaign in the financial industry as being positive to the market.
Hong Kong stocks also rose as investors consolidated positions ahead of a U.S. Federal Reserve policy decision later in the day. Some analysts said a rise in U.S. interest rates would remove a big uncertainty for investors and support the Hong Kong dollar, which is pegged to the U.S. dollar.
The CSI300 index rose 0.3 percent, to 3,320.54 points at the end of the morning session, while the Shanghai Composite Index gained 0.5 percent, to 3,168.31 points.
Surprising many investors, China's graft watchdog said late on Wednesday that the assistant chairman of China's securities regulator was under investigation for suspected "serious violation of discipline".
The announcement came a day after China's top brokerage CITIC Securities said its general manager and some other senior executives were being probed for alleged insider trading.
"Anti-corruption is good for the market in the long term," said Samuel Chien, a partner of Shanghai-based hedge fund manager BoomTrend Investment Management Co.
"In the short term, the market will likely be range-bound as investors watch what would happen next."
Analysts said that with main indexes now down roughly 40 percent from their mid-June peak, most of the grey-market leveraged positions unwound, and the use of shorting tools heavily restricted, much of the selling pressure had been released.
Small-caps continued to bounce sharply on Thursday morning, with the growth board ChiNext up 2.9 percent.
Shares of Chinese software makers, big-data companies, and firms in the business of Internet security surged, as investors bet President Xi Jinping's visit to Washington next week would put the spotlight on the two powers' tensions over cyber-security issues, creating thematic investment opportunities.
But blue chips, including banking shares, property plays and transport stocks sagged, reflecting lingering concerns over the health of the economy.
In Hong Kong, the Hang Seng index added 0.8 percent, to 22,136.36 points, while the Hong Kong China Enterprises Index gained 2.3 percent, to 10,135.33.
The U.S. Fed is considering raising U.S. interest rates for the first time since 2006, although economists are split in their expectations. The Fed's decision is due at 1800 GMT.
Sam Chi Yung, analyst at Delta Asia Financial Group, said Hong Kong stocks would benefit from a U.S. rate rise because the action would remove one of the biggest uncertainties facing investors, and would also help strengthen the Hong Kong dollar, which is pegged to the U.S. dollar. (Reporting by Samuel Shen and Pete Sweeney; Editing by Jacqueline Wong)