BRUSSELS—U.S. officials are preparing to test Moscow’s willingness to end the Ukraine conflict by seeking Russia’s approval for 20,000 peacekeepers across Ukraine’s embattled east, U.S. and Western officials said.

The U.S. is expected to raise the proposal with Russian officials in coming days, these people said. It is driven by hopes in some Western capitals, including Paris and Berlin, that Russian President Vladimir Putin is seeking a way out of Moscow’s military support to Ukrainian separatists.

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