Russian forces have the advantage on the battlefield, so a decision by Mr. Putin to settle rather than try to conquer more of Ukraine should be seen as a concession in itself, said Feodor Voitolovsky, director of the Institute of World Economy and International Relations, a Russian government-funded research group.

Even with Trump's direct involvement, Moscow's preconditions for a ceasefire in Ukraine make it a swift ending to the bloodshed difficult to reach.

The threat of more U.S. sanctions jump-started stalled peace talks but might not produce a deal to halt the fighting.