MOSCOW—President Vladimir Putin moved to pull Ukraine back from the brink of breakup Wednesday, calling on separatists to put off a secession vote, softening his attacks on a presidential election and saying his troops had withdrawn from the border.

Mr. Putin's change of tone came after weeks of escalation in the crisis over Ukraine, which has taken dozens of lives and brought East-West relations to their lowest point since the Cold War.

But Washington was dismissive and immediately questioned whether Moscow was genuinely committed to stabilizing Ukraine, where the government in Kiev is strongly pro-Western.

The Pentagon said it had seen no evidence to back Mr. Putin's claim that he had called back the tens of thousands of troops deployed to Ukraine's border last month.

The Obama administration noted Mr. Putin failed to call on the pro-Russian separatists to lay down their arms or vacate buildings they have commandeered in eastern Ukraine in the run-up to the referendum they plan to hold on Sunday.