Ukraine’s government has released photos that it says show Russian soldiers among militants holding official buildings in eastern region of the country.
Handed to the Organization for Security and Co-operation in Europe (OSCE) last week, the photos have since been distributed by the US state department.
They are said to show Russian soldiers or paramilitaries in flashpoint towns in Ukraine’s Donetsk region.
Russia denies it has military units on the ground in Donetsk.
Pro-Russian militants are holding official buildings in towns and cities in the east.
According to the Ukrainian press release, the photos show the same bearded gunman taking part in militant operations in the Donetsk towns of Kramatorsk and Sloviansk this year, and in operations in Georgia in 2008.
However, in the 2014 photos, his greying beard appears to be black while in Georgia six years ago, the slimmer-looking man shown has a reddish beard.
Other unverified photos are said to show the same masked gunman in both Donetsk and Crimea, the Ukrainian region annexed last month by Russia.
The Ukrainian press release argues that the photos are proof of a Russian special forces unit operating in eastern Ukraine.
Ukraine’s permanent representative to the International Organizations in Vienna said the photos provided “growing evidence of Russia’s involvement in instigating and co-ordinating the separatist actions that destabilise the situation in the east of Ukraine”.
US state department spokeswoman Jennifer Psaki said there was “broad unity in the international community about the connection between Russia and some of the armed militants in eastern Ukraine”.
“The photos presented by the Ukrainians last week only further confirm this,” she said, adding that it was a “pivotal period” for Russia to “use their influence to de-escalate the situation in Eastern Ukraine”.
Russia has previously denied it is destabilizing Ukraine, and warned the authorities in Kiev against any use of force against pro-Russian demonstrators.
“There are no Russian units, special services or instructors in the east of Ukraine,” President Vladimir Putin said on Thursday.
[youtube mr4_GZZaoXE 650]