Chinese thermal coal futures saw their biggest weekly fall in over five years on Friday as the powerful state planner said there was more room to adjust coal prices after its recent investigations into producers.

"Initial results showed coal production costs are significantly lower than current spot coal prices," the National Development and Reform Commission (NDRC) said in a statement.

The most-traded thermal coal futures contract on the Zhengzhou Commodity Exchange hit its lower limit of 10% before closing down 8.64% at 973 yuan ($152.24) per tonne. It was down 27.6% for the week, the biggest weekly decline since September 2016.

The contract has more than halved from its record high of 1 982 yuan on Oct. 19 but is still up more than 80% this year.

Physical coal prices have not dropped at the same speed, however. Spot prices at the port of Guangzhou, as assessed by China Chengxin Information Technology, slumped by almost 20% this week but are still twice as high as the futures price at around 2 000 yuan a tonne.

Traders have said they are scrambling for price information on spot coal transactions due to a lack of accurate quotations by index providers in China.

On Friday, the environment ministry said it would take steps to cut the concentration of tiny hazardous airborne particles known as PM2.5 by an average of 4% on the year in main cities this winter.

The world's top greenhouse gas emitter's anti-smog programme is likely to face scrutiny as a power crunch forces the government to secure more coal to generate electricity.

Measures to cool prices of the key power-generating fuel include stepping up output in coal producing regions and setting a target price of thermal coal.

The power crunch has forced power-hungry industries such as steel, aluminium, cement and chemicals, to cut production, while a new electricity tariff is expected to bring higher costs and put pressure on profit margins.