Since March, Putin has vowed to use military force to protect Russian-speaking "compatriots" across the former Soviet Union. He branded southern and eastern Ukraine "New Russia", a name the rebels took up as catch-all for most militia groups.

Most people in eastern and southern Ukraine are ethnic Ukrainians who speak Russian as their native language.

Polish Prime Minister Donald Tusk said on Wednesday the threat of a direct intervention by Russia's military in Ukraine has risen over the last couple of days.

A senior U.S. official said Washington was keeping a "very close eye" on Russian activity at the border.

"The last few weeks have not been good ones for the Russians. They have seen the separatists lose ground and the Malaysian airliner shoot-down was a public relations disaster for the Russians. Unfortunately we have not seen a sense that the Russians are acting on the negative feedback and looking to deescalate. In fact we've see signs that Putin seems to be doubling down," said the official.

"There's the buildup on the border, no change in the propaganda machine in Russia, talk of the need for humanitarian peacekeeping in Ukraine itself." (rt/ez)