An investor looks at an electronic board showing stock information at a brokerage house in Fuyang Thomson Reuters SHANGHAI (Reuters) - Senior executives of listed firms in China have stepped up the pace at which they are selling shares in their own companies, suggesting they may have doubts about whether their stock prices can go much higher.

In May company insiders - senior executives or their relatives - sold a combined 1.68 billion shares, a tripling from April, and much more than in each of the previous months of this year, according to data compiled by Reuters.

Share sales by senior managers are sometimes taken as a worrying sign by investors, as they can indicate people who know a company best think its stock price is too high.

"It's a vote of confidence," said Hong Hao, chief strategist at Bank of Communications International.

"It's evidence that stock prices are high".

However, Hong added he expects these sales will only slow, not stop, stock price rises, in what's become a frenzied market.

China's benchmark stock indexes have surged nearly 150 percent over the past year, beating the rest of the world's major indexes, even as the country's economy slows.

Shenzhen's start-up board ChiNext has more than tripled in the last 12 months and is now trading at earning multiples of 140, meaning at the current level of profitability, investors need to wait 140 years to recoup their investments.

Share holders observe the stock market at a stock exchange corporation on December 17, 2014 in Nantong, Jiangsu province of China. ChinaFotoPress/ChinaFotoPress via Getty Images

Some shareholders are getting impatient.

Between June 1 and June 3, Jia Yueting, Chairman and president of Leshi, sold 35 million shares in the internet firm he founded, making 2.5 billion yuan ($402.85 million).

Leshi said on May 25 that Jia plans in total to sell up to 148 million shares over the next six months or 8 percent of the company, though he will remain the biggest shareholder after that with a 36.85 percent holding.

Leshi said that Jia will lend the proceeds of the sale to the company interest free. A spokeswoman declined to say whether the selling was prodded by a view that share prices are too high.

Cashing out

It's not just company management who are selling; major cornerstone investors, freed from mandated lock-up periods, are also reducing their stakes.

Cornerstone stakeholders slashed 109 billion yuan worth of China-listed shares in May, double the amount sold during the previous month, according to data from Southwest Securities.

That doesn't include 3.5 billion yuan worth of banking shares dumped by sovereign investment firm Central Huijin on May 26.

The reduction by Huijin, in its holdings in China Construction Bank (CCB) and Industrial and Commercial Bank of China (ICBC), represents a reversal of strategy by the state investor.

Chinese national flag flies in front of the China Construction Bank (CCB) Tower at Hong Kong's business Central district December 26, 2014. REUTERS/Tyrone Siu

Huijin had been buying mainland-listed banking shares since the global finiancial crisis in 2008, in an apparent support to their share prices.

Huijin confirmed in statement that it has sold the shares but did not provide its reasons for doing so.

A similar trend was captured by an index compiled by Shenwan Hongyuan Securities that tracks major shareholders' trading activities.

The index surged over the past month, to a record high, meaning major shareholders are reducing holdings at unprecedented levels.

"It a barometer of how people in the real economy view stock valuations," said Liu Junwei, analyst at Shenwan Hongyuan.

"It means at the current level, a correction is very likely".

($1 = 6.2058 Chinese yuan renminbi)

(Editing by Rachel Armstrong)