Hundreds of U.S. troops are headed for maneuvers in Eastern Europe through year's end, the Pentagon announced, new deployments intended to reassure allies on Russia's borders as violence took a sinister turn Tuesday in embattled Ukraine.

Vice President Joe Biden, in a visit to Kiev, warned Russia to pull back its troops and abide by last week's international agreement or face the certainty of swift new sanctions. Mr. Biden also urged Ukraine's leaders to adhere to democratic principles and respect cultural differences, reassuring...