Shells of turtles which died of "natural causes" must not be used in traditional Chinese medicine, China's food and drug watchdog said Wednesday.

Local authorities are urged to strengthen supervision of the medicine market and confiscate all unqualified turtle shells, according to a circular issued by the China Food and Drug Administration.

Those found violating the regulation could be subject to criminal prosecution, the circular said.

Police in east China's Zhejiang Province recently detained a group of people who allegedly purchased and sold 40 tonnes of shells of dead turtles over many years.

The suspects admitted that they benefitted from the large amount and low cost of such shells in the black market, police said.

In traditional Chinese medicine, turtle shells can only be used after killing live turtles and removing the shell with boiling water. The medicine's main functions are to reinforce body fluids, nourish the blood and disperse nodules.