Authorities in southwest China have closed a number of Wal-Mart stores after they were found to be selling pork falsely labelled as organic, the US retail giant and state media said on Monday.

Investigators found 10 Wal-Mart stores and two Trust-Mart outlets owned by the US company in the mega-city Chongqing had been selling regular pork as the more costly organic pork for 20 months,China News Service said.

Some employees have been detained and the stores have been ordered to pay more than 2.69 million yuan ($420,000) in fines and repayments from illegal gains, it said.

The stores were ordered on Sunday to close for 15 days, Christina Lee, a Beijing-based spokeswoman for Wal-Mart, told AFP.

"We believe the closure has come about as a result of a recent investigation in some of our stores in Chongqing due to the 'green pork' incident by which the rights of consumers were infringed," she said.

"We have fully cooperated with the investigation and focused upon the fastest implementation of corrections suggested" by the local government, Lee said.

Wal-Mart opened its first stores in Chongqing in 2006 and has so far been punished 21 times for problems including selling food past its expiry date and false advertising, Chinese News Service said, citing an official.