Russia has retaliated against sanctions imposed by the European Union and the Unityed States, and will now ban imports of food products from the U.S., and all fruit and vegetable from the E.U.

The ban will impact on farmers in the West, and hit consumers in Russia who depend on cheap food imports.

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Moscow has been the biggest importer of European fruit and vegetables, and the second biggest buyer of U.S.-produced poultry after Mexico.

Last year, the Russians imported US$43 billion worth of food last year.

In 2011, Russian bought 28% of fruit exports from the European Union and 21.5% of its vegetables.

Russian news agency RIA said President Vladimir Putin has ordered the Russian food safety watchdog VPSS to draw up a list of imports that will be banned to punish those countries that earlier imposed economic sanctions on Russia.

Meanwhile, fighting has intensified in eastern Ukraine, while NATO fears a Russian invasion is imminent.

NATO spokeswoman Oana Lungescu said the organization is concerned about Russia having amassed around 20,000 troops on the Ukraine border.

"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground," the spokeswoman said.

A NATO military officer said the build-up included heavy artillery, tanks, air defense systems, aircraft, and special forces units.

Lungescu warned that Moscow could use "the pretext of a humanitarian or peace-keeping mission as an excuse to send troops" across the border into Ukraine.

Military officials in Kiev said their troops have been shelled from inside Russia. Eighteen Ukrainian servicemen had been killed and 54 injured in 25 separate clashes since Tuesday in eastern Ukraine.