BEIJING, July 8 -- China's state asset regulator ordered the country's centrally administered state-owned enterprises (SOEs) not to sell shares of their listed companies amid "abnormal market volatility" in an effort to stem massive stock market sell-offs.

The State-owned Assets Supervision and Administration Commission also encouraged SOEs to purchase more shares to stabilize prices.

The decision on Wednesday followed a raft of other supportive measures aimed at stemming a market nosedive that has seen the key Shanghai stock index plunge by more than 30 percent from its June peak, but the effort appears to have had no effect so far.

In the previous trading days, China's institutional investors spent billions buying shares of heavyweights such as Sinopec and PetroChina in an effort to restore market confidence, but the move only resulted in a short-lived rally.

The benchmark Shanghai Composite Index sank 6.97 percent to open at 3,467.4, while the Shenzhen Component Index opened 4.44 percent lower at 10,870.14 on Wednesday.

Immediately after the market opening, the central bank issued a statement reiterating its liquidity support to stabilize the market and avoid systematic and regional financial risks.

China Securities Finance Corporation Limited, the national margin trading service provider, pledged on Wednesday to purchase more shares of small- and medium-sized listed companies to ease stock market liquidity.

The China Securities Regulatory Commission (CSRC), the country's securities regulator, publicized a notice on Wednesday encouraging major shareholders, directors, supervisors and senior managers of listed companies to buy more stocks of their own companies to stabilize share prices.

The CSRC has temporarily allowed them to purchase their companies' stocks through securities and fund-management companies even if they have sold shares during the past six months. They have been permitted to buy more stocks if the share prices of their companies have dropped more than 30 percent during the past 10-day trading session on the condition that they promise not to sell for half a year.

The CSRC has also allowed a shareholder who owns more than 30 percent of a company's stocks to buy at most two percent more of its shares annually, according to the notice.

The adjustments came into effect on Wednesday.

So far, 111 centrally administered SOEs have already promised in a joint announcement not to sell shares in their listed companies.