MOSCOW (Reuters) - Meat imports to Russia from producers using ractopamine must be tested and certified free of the feed additive, the country's veterinary regulator said, with Moscow's health watchdog denying the requirement is a political retaliation.

The move, announced a day after the U.S. Senate approved a bill to expand trade between Washington and Moscow that also sought to punish Russian human rights violators, could jeopardize North American meat beef and pork suppliers.

It would potentially make the United States, which exports more than $500 million a year worth of beef and pork to Russia, significantly less competitive, giving advantage to Chinese and European Union meat producers, where ractopamine is banned.

The U.S. Meat Export Federation said the U.S. Department of Agriculture had no testing and certification program in place for ractopamine.

Russia's plant and health regulator, Rosselkhoznadzor, said that as of Friday it would allow for an unidentified transition period during which in the absence of a needed certification, Russia will test each shipment itself.

"During this period the veterinary service of the suppliers have to create a system of laboratory testing of products certifying the absence of ractopamine," the regulator said in a statement posted late Friday on its website.

Analysts said the Russian move was linked to the U.S. Senate's passage of the "Magnitsky Act" as part of a broad trade bill, which drew an angry response from Russia where officials called it "absurd."

Gennady Onishchenko, Russia's chief health inspector and head of the state consumer protection agency Rospotrebnadzor, denied the requirement of testing and certifying meat imports for ractopamine was retaliatory.

"In Russia, (ractopamine) is not included in the register of products approved for use," Onishchenko told the news agency Interfax on Saturday.

"We can only regret that American Federation analysts on meat exports lacked even a tiny bit of imagination to classify the 27 countries of the European Union, China and all other 167 countries that have banned the use of this product as opponents of the 'Magnitsky Act' adopted by the U.S. Senate."

Ractopamine is used as a feed additive to make meat leaner, but countries such as China have banned its use despite scientific evidence that it is safe. The United Nations has agreed on acceptable levels of the drug.

(Reporting by Lidia Kelly; editing by James Jukwey)