Defiant Russia, which has banned all U.S. food on its land, as well as fruits and vegetables from the EU in response to sanctions, is feared to invade Ukraine anytime soon. NATO said around 20,000 combat-ready Russian troops are now present on Ukraine's eastern border.
"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground - and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," Oana Lungescu, NATO spokeswoman, said in a statement.
The troop presence included tanks, infantry, artillery and air defense systems, Reuters quoted an unidentified military officer. There were also logistics troops, special forces and various aircraft.
Lungescu said Russia would most probably use as reason "a humanitarian or peace-keeping mission" that's why it is sending troops into Eastern Ukraine.
Fighting in eastern Ukraine since July 17 when Malaysia Airlines MH17 was downed by a missile by pro-Russian rebels had further escalated "a dangerous situation."
Early this year, Russia had placed some 40,000 troops close to the Ukraine border but pulled them out in June, dropping to less than 1,000.
But Russia started building the force up again, NATO said.
Chuck Hagel, the US defence secretary, confirmed NATO's worst fears.
"When you see the build-up of Russian troops and the sophistication of those troops, the training of those troops, the heavy military equipment that's being put along that border, of course it's a reality, it's a threat, it's a possibility - absolutely," he was quoted by the Daily Telegraph.
On Thursday, President Vladimir Putin ordered the ban of food and agricultural products against countries that had imposed sanctions against Russia. The restrictions will apply for a year.