Investigations are being conducted into alleged market manipulation of 18 A-share issues, the China Securities Regulatory Commission said on Friday.

The CSRC's announcement came after equities hit a four-year high on Friday, with the benchmark Shanghai Composite Index rising 1.67 percent to 3,108.6 points.

Mainland stocks have surged this year in highly volatile trading, triggering concern about speculative trading.

Zhang Xiaojun, a CSRC spokesman, told a news conference that the agency had noticed "new trends" in market manipulation that are especially negative for small investors. Such trading "has become much quicker and harder to detect", Zhang said.

The regulator will step up its crackdown on illegal trading activities, Zhang added.

Common tactics cited by the CSRC include high-frequency trading, placing fake orders and the rapid entry and cancellation of large orders to gain an advantage over slower market participants.

Zhang said that market manipulation is now often disguised by "value management", through which listed companies selectively disclose information to influence stock prices.

Most of the 18 stocks being investigated are listed on the board for small and medium-sized enterprises at the Shenzhen Stock Exchange, a market that is prone to manipulation due to the relatively small market capitalization of the stocks.

The equity market in the Chinese mainland has long been seen by experts as more of a casino than a legitimate financing venue. The market has been plagued by financial scandals, insider trading and manipulation.

Earlier this month, the regulator warned of growing speculative risk after the market surged by 18 percent in just two weeks.

The CSRC ordered stock brokerages to give appropriate guidance to clients, and it also warned retail investors not to engage in risky practices such as "borrowing money or selling houses to buy stocks".

Those warnings have done virtually nothing to rein in gains or reduce volatility.

The Shanghai index jumped as much as 2 percent and dropped as much as 1.3 percent on Friday. Price swings have increased, with 30-day volatility surging to the highest level since November 2010 this week, Bloomberg reported.

The Shanghai market has gained 47 percent this year, sending valuations to 11.6 times 12-month projected earnings, the highest level in three years, according to data from Bloomberg.

Investors are betting on further monetary loosening by the People's Bank of China to shore up the economy. Many believe that the central bank will reduce banks' reserve requirement ratios as a follow-up to last month's interest rate cut, the first in two years, to support growth.

Analysts said that falling oil prices will continue to benefit sectors such as airlines and shipping lines in China, so that bullish sentiment will push stock prices higher but at a more stable pace.

"The market is trending up at a measured pace now," Dai Ming, a fund manager at Hengsheng Asset Management Co, was quoted by Bloomberg as saying.