WASHINGTON — President Trump cast doubt on whether Moscow is backing separatists engaged in the recent escalation of fighting in eastern Ukraine, appearing to side with President Vladimir V. Putin of Russia, who has long denied involvement in the conflict despite evidence to the contrary.

Mr. Trump said he did not take offense at the outbreak of a lethal bout of fighting in Ukraine that came within a day of a phone conversation he had with Mr. Putin, saying of the recent clashes, “we don’t really know exactly what that is.”

“They’re pro-forces,” Mr. Trump said of the Ukrainian separatists in an interview that aired on Monday on “The O’Reilly Factor,” on Fox News. “We don’t know, are they uncontrollable? Are they uncontrolled? That happens also. We’re going to find out; I would be surprised, but we’ll see.”