KIEV, Ukraine—President Viktor Yanukovych bowed to pressure from the West and a widening rebellion at home by agreeing Wednesday to a truce with demonstrators who spent the day staring down riot police across a smoldering no-man's-land in the center of the capital.

The move away from confrontation came as the U.S. and Europe—blindsided by the violence that left at least 25 people dead—edged toward imposing sanctions on Ukrainian officials they deem responsible for the crackdown.

...