China's Ministry of Commerce said Friday that it has asked local departments to crack down on illegal oil sales, compulsory tie-in sales and overcharging, as seasonal demand for oil will increase as summer arrives.

The ministry said in a notice that some oil sellers have violated laws and regulations, hurt consumer interests and disturbed market order. Local authorities should enforce market admittance and ban illegal trading to ensure a safe and orderly market environment, the notice said.

Serious measures will be taken to find and punish sellers who dilute oil with other substances such as ethanol and methanol, the notice said.

The notice also requires local departments to ensure ample market supplies and avoid shortages.

Oil prices fell below 109 U.S. dollars per barrel on Thursday, extending a weeklong sell-off amid concerns that slower economic growth in the U.S. will undermine demand for crude oil.

According to China's oil price-fixing mechanism, which was updated in 2009, domestic oil prices can change after international crude prices rise or fall by more than 4 percent over 22 working days.