China has pledged to investigate allegations that capsules containing the powdered flesh of dead babies are being produced on its soil and smuggled into South Korea.

The gruesome practice came to light Sunday when Korea Customs said it had uncovered multiple attempts to illegally import, in total, more than 17,000 of the capsules in travellers' luggage or by mail.

The pills are said to be filled with the flesh of foetuses or dead infants, dried then ground into powder, to be taken as a disease cure or to boost sexual performance.

Beijing said a previous investigation into similar allegations had uncovered no evidence that such capsules were made in China, but pledged to reopen the investigation.

"We have not yet found the relevant capsules in China," foreign ministry spokesman Hong Lei said Wednesday.

"The health ministry will further investigate (this) in conjunction with relevant public security, industry and commerce and customs departments."

Experts say the practice stems from a superstitious belief that eating the body parts of young infants will impart special physical strength or cure disease.

But aside from the obvious ethical issues, there are worries the capsules could be contaminated with "super bacteria" and other disease-causing organisms.

China's health ministry declined to comment Wednesday when contacted by AFP, but spokesman Deng Haihua told the Shanghai Daily that reports of the capsules first surfaced in South Korean media in August last year.

A Korea Customs official told AFP on Tuesday that the country would closely monitor flights from "certain Chinese regions" and inspect the luggage of passengers more frequently than before.

Korea Customs said pills were sent from at least four Chinese cities at the request of customers in South Korea, but were intercepted in the mail or in customs searches at airports.

They came from Jilin and Yanji cities -- both in the northern province of Jilin which borders North Korea -- as well as the northern municipality of Tianjin and eastern city of Qingdao, it said.

Some were hidden in packages of legitimate drugs to disguise their contents.

Bringing in such pills breaches a regulation banning items that "violate social dignity and customs", said Kim Soo-Yeon, an official in charge of customs clearance.

The capsules sell for 40,000-50,000 won ($35-$44) each at some herbal medicine shops, South Korean media says.

Chinese hospitals cannot dispose of foetuses and deceased infants as medical waste, the Shanghai Daily said. They must be treated as other human remains and cremated.

Under Chinese law, medical institutions are also forbidden to trade in foetal remains or placentas, the Global Times, an English-language Chinese newspaper said.

But Chinese hospitals do allow mothers who have just delivered to take their own placentas, provided they sign an authorisation form, a Shanghai maternity hospital said.

One Chinese health website posted recipes for both human and animal placenta, including soup, dumplings and meat balls.

"Many people with weak constitutions want a quick way to take human, cow, sheep and other animals' placenta, and process it for food," said website www.fh21.com.cn.