The US State Department has released images - provided by Ukraine - which it says proves armed separatists in eastern Ukraine are actually Russian military or intelligence officers.
It is the latest attempt by the US to disprove Moscow's claims that it has nothing to do with pro-Russian separatists.
State Department spokeswoman Jen Psaki said as well as those from the Ukrainian authorities, images widely available in the international press or on Twitter indicate the men are members of the Russian forces.
They show "signs of a connection ... between Russia and some of the armed militants in eastern Ukraine", said Psaki.
"These are just further evidence of the connection between Russia and the armed militants," she added.
A series of photos provided by Ukraine to Washington through the Organisation for Security and Co-operation in Europe shows a photograph taken in Georgia in 2008 of a man with a distinctive long red beard wearing the insignia of a Russian special forces unit.
The same man - with the beard a little greyer - can be seen in a photo taken at the recent capture of a police station in the Ukrainian town of Kramatorsk by separatist militants, and again in Slavyansk last week.
Another series of images, some taken in Crimea, appear to show the same man once more, while more photographs from Slavyansk show men carrying the same kind of RPG-26 rocket launchers as that issued to Russian troops.
Last month, Russia annexed the Crimean peninsula, which had been Ukrainian territory.
On his official blog, General Philip Breedlove, Nato's supreme allied commander in Europe, outlined why he believes it is clear the men in the photos are from the Russian forces.
He explained: "The weapons and equipment they carry are primarily Russian army issue. This is not the kind of equipment that civilians would be likely to be able to get their hands on in large numbers."
The accusations come amid increasing pressure on the White House to come up with a more forceful response to Russia's role in the crisis.
Former Republican presidential nominee Bob Dole said on Monday that the US should send weapons, including tanks, to Ukraine to help it resist Russia's moves on its territory and to send Russian President Vladimir Putin a strong message.
The 90-year-old Republican Party icon, a former party leader in the US Senate, said Putin "has sort of sized up" Democratic President Barack Obama and "concluded that he's not a strong leader".
Dole's comments came as US Vice-President Joe Biden began a high-profile visit to Ukraine.
They also came just days after an announcement in Geneva that talks between Ukraine, Russia, the US and the European Union had produced an agreement to take tentative steps toward calming the volatile situation in eastern Ukraine.
The accord was meant to reduce tensions in the worst confrontation between Washington and Moscow since the cold war.
US Secretary of State John Kerry called on Moscow to put pressure on the pro-Russian separatists, which Washington sees as backed by Russia.
However, Russian Foreign Minister Sergey Lavrov urged Washington to influence the Ukrainian government, which Moscow accuses of "grossly breaching" the Geneva deal.
Kerry told Lavrov that "concrete steps" to defuse the crisis should include "publicly calling on separatists to vacate illegal buildings and checkpoints, accept amnesty and address their grievances politically", the State Department's Psaki said.
Biden yesterday met Ukraine's new pro-Western leaders in a symbolic show of his country's support for Kiev.
Additional reporting by Associated Press