Stock investors beware: big sell orders in China could land you in trouble with the authorities.

That’s the message from the Shanghai Stock Exchange, which said on its microblog Monday that two trading accounts got verbal warnings for a “large amount of sell orders affecting security prices or volume.” The bourse said the trading was “abnormal,” but didn’t give any details on the two accounts or indicate whether any laws were broken.

The warnings follow investigations into algorithmic trading and short selling, part of a government campaign to prop up share prices and prevent market manipulation after an almost $4 trillion selloff. While proponents of the intervention say it’s necessary to restore investor confidence, critics have argued that China is backtracking on its pledge to give markets more sway in the world’s second-largest economy.

“Investors will feel it’s not an international standard,” said Steven Leung, an executive director at UOB Kay Hian Ltd. in Hong Kong. “If investors think the market is coming down, of course they will place sell orders.”

The exchange also issued three warnings to accounts that had “frequent cancellations of orders involving large amounts,” it said in the posting after the close of local markets on Monday. The latter warnings are consistent with the bourse’s previously announced investigation of spoofing, a practice that involves placing then canceling orders to move prices.

‘Window Guidance’

The Shanghai Composite Index fell 1.1 percent on Monday, extending its decline from a June 12 high to 30 percent.

Given the limited details in the Shanghai exchange’s statement, it’s unclear what about the sell orders elicited a warning from the bourse, said Tony Hann, a portfolio manager at Blackfriars Asset Management in London. A spokesman from the Shanghai Stock Exchange who asked not to be identified because of bourse policy said he couldn’t immediately comment on the warnings.

“If the move is against market manipulation, then the move is justified in any environment,” Hann said. “Without more details, we’re working in the dark.”

For Wu Kan, a Shanghai-based fund manager at JK Life Insurance Co., the warnings on sell orders are equate to “window guidance” from authorities.

“It’s an unconventional strategy that’s used in a difficult time,” Wu said. “We are still in the stage of rescuing the market, and the regulators will try every possible means to stabilize the market.”

Regulators are probing “malicious” short selling and have examined the futures trading accounts of foreign investors. They’ve also banned stock sales by major shareholders, suspended initial public offerings and compelled state-run institutions to support the market with equity purchases.