China's top planning body has asked the country's coal firms to keep coal prices stable as the economy is facing an "arduous task" to check inflation.

Coal companies were required to keep prices unchanged for coal under annual supply contracts, and those that have increased term coal prices must reduce prices before the end of June, said Cao Changqing, chief of the department of price with the National Development and Reform Commission (NDRC).

"Rising coal prices would affect downstream businesses and also lift the country's inflation expectation," said a statement posted on the website of NDRC.

State-owned coal enterprises and other leading coal firms should take the lead in keeping coal prices stable, the statement noted.