The Dalian Commodity Exchange said it would double the transaction fees on steelmaking raw materials coking coal and coke futures from Thursday, the fourth increase in a week.

Dalian has also raised the transaction fees on iron ore futures twice this week as the exchange, along with other commodity platforms in Shanghai and Zhengzhou, imposed curbs to restore calm in markets after a week-long surge unsettled global investors.

The curbs implemented by the exchanges seemed to be having the desired effect, said Wang Di, analyst at CRU consultancy in Beijing.

"What they're trying to do is minimise too much speculation, especially from the retail investors."

Analysts say speculators were betting that a rise in infrastructure spending in China would lift raw material prices, which have been depressed for years by a persistent supply glut.

The rapid price gains have defied the supply-demand fundamentals of the commodities, and analysts warn there is a risk of a bigger correction ahead, similar to last summer's crash in Chinese shares, which also followed a frothy run-up.

"If prices in China go too far off from fundamentals, markets in China could lose credibility, similar to what had happened in the stock market," Citigroup analysts said in a report.

The most traded September iron ore on the Dalian Commodity Exchange dropped by its 6 percent downside limit for a second day in a row on Wednesday, before closing 5.4 percent lower at 434.50 yuan (46 pounds) a tonne.

Coking coal and coke on Dalian also slid by the 6 percent maximum allowed by the exchange, but coke slightly trimmed losses to end down 4.9 percent.

On the Shanghai Futures Exchange, rebar - or reinforcing bar used in construction - fell 3.7 percent to 2,500 yuan a tonne . The less-traded hot rolled tumbled by its 6 percent downside limit before closing 4.6 percent lower.

After raising the transaction fee on iron ore futures contracts twice, the Dalian exchange said it "will step up supervision and resolutely curb signs of overheated speculation in some products to prevent risks and maintain steady operations of markets".

Wednesday's pullback was concentrated on steel and steelmaking futures, but other commodities also slipped. Cotton on the Zhengzhou Commodity Exchange and egg futures on Dalian each fell nearly 2 percent.

(Reporting by Manolo Serapio Jr.; Additional reporting by Ruby Lian in Shanghai; Editing by Will Waterman)

By Manolo Serapio Jr