MUNICH—The U.S. and Germany are struggling to maintain a united front against an unflinching Russia ahead of a crucial week of high-stakes, top-level diplomacy on the Ukraine crisis.

Chancellor Angela Merkel has given Russian President Vladimir Putin until Wednesday to agree to a road map to end the fighting in eastern Ukraine, according to Western officials. If in her assessment Russian intransigence has blocked a deal, they said, Germany will likely move to step up European sanctions against Russian companies, possibly including...