The U.S. and Europe stepped up warnings Friday of additional sanctions on Russia unless cease-fire violations in eastern Ukraine stop, as the government in Kiev said more Russian tanks and other weapons were crossing the border to bolster pro-Moscow separatists.

The peace plan was brokered by the leaders of France and Germany in Minsk, Belarus, on Feb. 12, but the first step—a truce—has yet to fully take effect.

Officials...