Ukraine may be "most significant conflict" since WWII: analyst

"In many ways, this is President Putin going back to address the outcome of the Cold War," Andrea Kendall-Taylor of the Center for a New American Security (CNAS) told Reuters TV. "I think he's looking to rewind time, to rewrite the rules of the European security order....So it's about Ukraine, but it's also about much more than Ukraine," she said.

Tensions remained high after NATO said on Monday it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets in response to a Russian troop build-up near its border with Ukraine.

Russia, which denies planning an attack, said it was watching with "great concern". Kremlin spokesman Dmitry Peskov repeated Moscow's line that the crisis is being driven by U.S. and NATO actions, not the Russian troop build-up.

The United States and the European Union have threatened economic sanctions if Russia launches an invasion and Western leaders say unity is paramount, though differences have emerged among European nations over how best to respond.