MOSCOW — With a referendum on secession looming in Crimea, Russia massed troops and armored vehicles in at least three regions along Ukraine’s eastern border on Thursday, alarming the interim Ukraine government about a possible invasion and significantly escalating tensions in the crisis between the Kremlin and the West.

The announcement of the troop buildup by Russia’s Defense Ministry was met with an unusually sharp rebuke from Chancellor Angela Merkel of Germany, who warned that the Russian government must abandon what she called the politics of the 19th and 20th centuries or face diplomatic and economic retaliation from a united Europe.

“Ladies and gentlemen, if Russia continues on its course of the past weeks, it will not only be a catastrophe for Ukraine,” she said in a speech to the German Parliament. “We, also as neighbors of Russia, would not only see it as a threat. And it would not only change the European Union’s relationship with Russia. No, this would also cause massive damage to Russia, economically and politically.”

Ms. Merkel’s words reflected the rapid evolution of the Ukraine crisis from a regional conflict to a full-blown East-West confrontation that threatens a deep rupture in relations between Moscow and an increasingly unified European Union and the United States. That a leader of Germany, which has traditionally sought to bridge the East-West divide, should speak so forcefully was a further indication of the seriousness and depth of the potential breach.