File photo of a man working in his shop selling pork products in Shanghai. [Photo / Agencies]

China wants a third party to verify beginning March 1 that US pork shipped to the country is free of a feed additive used to promote lean muscle growth, a US Meat Export Federation spokesman told Reuters.

The step comes on the heels of Russia barring imports of US meat worth $550 million a year due to the same feed additive.

Officials from the China's quarantine bureau, which oversees the safety of food imports, declined to make immediate comment, while a spokesman said the country's commerce ministry was unaware of the move.

There was concern that China's requirement for third-party testing could hurt US pork exports to the Chinese mainland and Hong Kong, valued at $886 million last year.

"We have just been notified (by US suppliers) and are checking details with the quarantine authorities," said a trader with a large State-owned pork importer in China.

Beijing maintains that there are serious concerns about ractopamine, despite scientific evidence that it is safe. The United Nations has agreed on acceptable levels.

The quarantine bureau in May rejected a consignment of US pork after tests found traces of the drug.