WASHINGTON (AFP) - US Secretary of State John Kerry warned Russia on Tuesday against any military incursion into eastern Ukraine and likened the "nationalistic fervour" fuelled by the Crimea crisis to the build-up before World War II.
Any moves by Russia to march into eastern Ukraine "would be as egregious as any step that I can think of that would be taken by a country in today's world, particularly by a country like Russia, where so much is at stake," Mr Kerry said.
"Now I hope we don't get there," he told a gathering of students at the State Department, adding it "would be just an enormous challenge to the global community and it would require a response that is commensurate with the level of that challenge."
The top US diplomat said he did not want to start laying out options for any US or global response to any Russian move into eastern Ukraine "until we measure where we are" after Moscow absorbed Crimea into the Russian Federation.
"Today is egregious enough when you raise this nationalistic fervour which could in fact infect in ways which could be very, very dangerous," Mr Kerry said.
"All you have to do is go back and read in history of the lead-up to World War II and the passions that were released with that kind of nationalistic fervour.
"And obviously there's a tough history of things like Czechoslovakia in 1968, where the alleged rationale for going into the country was to protect the people in it," he said.
"You can ask the Poles how they felt being 'protected' for all those years," Mr Kerry added.
The United States on Monday unveiled a slew of sanctions against seven Russians in President Vladimir Putin's inner circle and warned of further moves.
"There are more to come," warned White House spokesman Jay Carney, condemning Russia's "moves to formally annex" Crimea.
US President Barack Obama has called for a G7 summit next week in The Hague to discuss the escalating showdown, as the interim leaders in Kiev authorised their troops to shoot in self-defence after both sides suffered their first casualties since pro-Kremlin forces seized Crimea nearly three weeks ago.
"We are preparing additional sanctions," confirmed State Department spokesman Jen Psaki.
"The power of the executive order yesterday is it provided us additional flexibility on who we would be able to sanction, what institutions we would be able to sanction."
Mr Putin signed a treaty claiming Crimea as Russian territory after the Black Sea region overwhelmingly voted in favour of switching from Ukrainian to Kremlin rule.
While Putin has so far not been hit with any sanctions, Ms Psaki said: "I'm not ruling anyone in or out. But the executive orders that have been signed provide broad authority."
Mr Kerry said comments from Putin about what was happening in Crimea "just didn't jive with reality or with what's happening on the ground".
"And the President may have his version of history, but I believe that he and Russia, for what they have done, are on the wrong side of history."