Do a series of photos of gun-toting men wearing green uniforms prove Russian forces are operating in eastern Ukraine?
Ukrainian officials point to the pictures in a dossier obtained Monday by CNN, arguing that the images show Russian "sabotage-reconnaissance groups" acting in Ukrainian towns.
The images, Ukrainian officials say, prove organized Russian activity in the region.
CNN cannot independently confirm the photographs, some of which were first published in The New York Times.
The dossier shows what Ukrainian officials say are images of well-equipped gunmen in eastern Ukraine who look similar to photographs of Russian forces taken in Crimea, Russia and during Russia's 2008 invasion of Georgia.
Last week, Ukrainian security officials told CNN they had arrested a Russian military officer and a woman Ukrainian officials said is a Russian intelligence agent.
Moscow has disavowed involvement in the takeover of government buildings in eastern Ukraine or other acts by often-masked pro-Russian gunmen.
But the photos, accepted as genuine by the Obama administration, appeared to lend credence to allegations by Ukrainian officials that Russian forces have been dispatched in eastern Ukraine to provoke a military confrontation.
If genuine, the photos also back up Western leaders who have claimed Russia's involvement. Last week, NATO commander Gen. Philip Breedlove wrote on the alliance's blog that what pro-Russian groups have described as an organic uprising is in reality a "well-planned and organized" military operation orchestrated by Moscow.
"There has been broad unity in the international community about the connection between Russia and some of the armed militants in eastern Ukraine, and the photos presented by the Ukrainians last week only further confirm this, which is why U.S. officials have continued to make that case," State Department spokesman Jen Psaki told CNN on Monday.
The question of whether Russia is involved in the unrest roiling eastern Ukraine is crucial as European observers try to enforce an agreement reached last week to lower tensions in the region by organizing the withdrawal of forces from government buildings and other facilities.
Ukraine provided the photos to those observers from the Organization for Security and Cooperation in Europe, according to the briefing dossier obtained by CNN's Elise Labott.
One image shows a gunman with a long beard during an assault on a police station in Kramatorsk who appears similar to a Russian special forces member photographed during the Russian invasion of Georgia.
Another shows gunmen photographed occupying administrative buildings in Slaviansk who appear similar to men pictured in what Ukrainian officials described as a "family photo" of a Russian sabotage and reconnaissance unit.
Natacha Rajakovic, deputy spokeswoman for the Organization for Security and Cooperation in Europe, declined to comment on the images and referred questions to Ukrainian authorities.
Ukrainians maintain that Russian involvement in the east is widespread.
CNN has heard Russian accents among the "men in green," as they are known, well armed and uniformed groups who have appeared in towns like Slaviansk and Kramatorsk. One told CNN he had come up from Crimea. But CNN has not seen any evidence that these men are operating under orders from the Kremlin.
Russia's Foreign Minister scoffed at the accusations, saying that Kiev and its patrons, the United States and the European Union, are trying to blame his country for everything.
Slaviansk's self-declared mayor, a former military man himself, says the explanation is simple: He put out an appeal to his old comrades.
"When I called on my friends, practically all of whom are ex military, they came to our rescue, not only from Russia but also from Belarus, Kazakhstan and Moldova," he said.