A notice on the State Food and Drug Administration's (SFDA) website posted late on Monday gave the company's name in Chinese as the Singapore Xin Ri Pharmaceutical Factory, and said it sold the pills under the name 'Tian Can Zhen Tong Pian', which translates as 'Silkworm pain-relieving tablets'.

The SFDA statement said the company is 'suspected of illegally adding and increasing the anti-inflammatory and pain-relieving chemical compound diclofenac' in its tablets which have been marketed as containing only Chinese medicine such as herbs.

The watchdog said it found that 19 batches of the pills contained diclofenac and 'according to China's drug supervision regulations, these batches are considered to be fake drugs'.

Diclofenac is a generic painkiller used commonly in Western drugs to reduce inflammation and treat minor aches and pain.

The SFDA did not say if the pills had caused any illness, or if consumers had complained. Asked if this was the first time it was taking such action against a Singapore drug firm, an SFDA spokesman would not say anything other than that the matter is under investigation.

A search on the Internet turned up the company's agent in China - Quanzhou Straits Drug Company, whose headquarters are in southern Fujian province and which has branches around the country including in Beijing, Tianjin, Shijiazhuang city in Hebei province and Guangzhou.

A supervisor in the Quanzhou Straits office in Guangzhou told The Straits Times that he was informed by head office early last week to stop sales of the pills as they were found to contain Western medicine compounds.

The supervisor, who gave his surname as Zhou, said he had not received any complaints since his firm started selling the pills last year.

'People say the pills are very effective,' he said.

An online review on the pills claims that two tablets once or twice a day can 'relieve pain, produce calm, invigorate blood circulation and dilate blood vessels' and can be used to 'relieve headaches and ailments caused by cold and wind'.

It listed eight Chinese herbs and medicinal silkworms as the tablets' key ingredients.

Sales staff at a large pharmacy chain in Beijing said that they had stocked the pills but had stopped selling them recently. A box of 30 pills costs 26 yuan (S$5).

China is stepping up its supervision on drugs sold in the country in an effort to restore confidence, internationally and at home, in its problematic drug safety system.

The SFDA said on its website on Monday that it has rejected a protein powder imported from a US supplier because it contained too much selenium, a trace mineral which can be toxic in large amounts.

ADDITIONAL REPORTING BY LINA MIAO