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 broader asset freezes.

Separately, the U.S. is considering supplying Ukraine with lethal aid. President Barack Obama has held off on a decision until he sees Ms. Merkel—who has publicly opposed weapons deliveries—on Monday morning.

The confluence of events—which follows a recent surge of deadly fighting between Ukrainian forces and Russian-backed rebels—has given that previously scheduled White House meeting potentially momentous import.