The masked men in military uniform spotted all over Ukraine's Crimean peninsula are not Russian military forces but "well-trained militia forces," Russian officials have claimed to America's top military officer.

U.S. Defense Secretary Chuck Hagel and Joint Chiefs Chairman Gen. Martin Dempsey aren't buying the claim, telling a congressional panel Wednesday that they are confident the masked men are Russian troops.

"It's pretty clear that they're Russian troops," Hagel told the Senate Armed Services Committee.

"My judgment is they are soldiers," said Dempsey.

Earlier in the day, Dempsey added, he had spoken by phone with his Russian counterpart, Gen. Valery Gerasimov, who denied that the military men spotted throughout Crimea were Russian military forces.

He said Gerasimov told him "that they were not regular forces; they were well-trained militia forces responding to threats to ethnic Russians in the Crimea."

Dempsey told the committee that he feels they are in fact Russian military forces.

"My military judgment is that these are soldiers who have been taken out of their traditional uniforms, repurposed for placement in the Crimea as a militia force," said Dempsey. "But my judgment is, they are soldiers."

However, Dempsey said, there is not concrete evidence to support his belief that they are Russian military forces in unmarked uniforms.

"We don't have any evidence as yet," he said. "I think evidence could likely become available over time."

Sen. John McCain, R-Ariz., complained to Hagel that U.S. intelligence had sustained what he called a "massive failure" for not predicting that Russian President Vladimir Putin intended to intervene militarily in Crimea.

"Mr. Putin was not going to see Sevastopol go into the hands of a government that was not his client, and that's a just a fact, " said McCain.

Hagel said the Obama administration was not caught unawares.

"We were well aware of the threats," Hagel said, citing NATO meetings in Brussels he attended last week and his recent conversations with Ukraine's previous defense ministers.

Either way, Hagel said, American officials don't place much trust in President Putin's claims about Crimea. Hagel said he agreed with Secretary of State John Kerry that "we don't accept anything that President Putin said as fact about why they had to protect the so-called ethnic minority in Crimea, and the other reasons that the Russians have laid out as to why they took the action they did.

Dempsey told the panel that he could not be certain where the Russian military forces inside the Crimea had come from, though the U.S. has been tracking Russian military exercises east of Ukraine's borders.

U.S. officials have also been unable to cite with precision exactly how many Russian forces entered Crimea.

One official said that he has seen estimates ranging from between two to ten thousand additional forces had been sent to Crimea. The official said it has been difficult to get a clear picture of what has been going on inside the region.

Another official said that the number of Russian military forces already stationed inside Crimea before the crisis developed last week was 15,000.