The Pentagon says there is little doubt that Moscow is supporting separatists responsible for a surge of fighting in Ukraine.

"That has been our assessment for a long time," said Navy Capt. Jeff Davis, "that Russia has its fingerprints all over what is going on in eastern Ukraine."

The unequivocal Pentagon statement stands in contrast to comments President Trump made in an interview that aired on Fox News Monday night, in which he indicated he was unconvinced that Russia was behind the latest flare-up of fighting in Ukraine.

Trump told Fox host Bill O'Reilly he was not insulted that within 24 hours of his phone conversation with Russian President Vladimir Putin, pro-Russian forces stepped up attacks in Ukraine.

"No, I didn't because we don't really know exactly what that is. They're pro forces. We don't know if they're uncontrollable," Trump said. "Are they uncontrollable? That happens also. We're going to find out. I will be surprised, but we will see."

Trump's measured comments drew praise from Russian Foreign Minister Sergei Lavrov, who said the tone was an improvement over the Obama administration.

"He wants to get to grips with everything in Ukraine and understand how to behave," Lavrov said of Trump.

"This, I already believe, is a big and qualitative shift compared to the Obama's administration," Lavrov said according the TASS news agency.

The Pentagon said while it is troubled by the surge of fighting in Ukraine, "what we haven't seen is any sort of large-scale movement of Russian forces that would suggest that this is part of something bigger," Davis said. "This is a flare-up of violence. It is a significant flare-up, the most significant we've seen since 2015."

Ukraine has been battling Russian-backed separatists nearly three years and an estimated 10,000 people have died in the war.

The Pentagon on Tuesday said it remains convinced there is no military solution to the conflict and that the Minsk agreement is the only way to resolve the conflict peacefully.

The U.S. has about 350 troops in Ukraine training and advising government security forces, and has provided $335 million in security assistance but has stopped short of providing lethal aid to the Kiev government.