The US State Department has released photos of soldiers in eastern Ukraine, which it says show that some of the fighters are Russian special forces.
The BBC is unable to verify the pictures, which were provided by Ukrainian diplomats.
The photos appeared to identify Russian soldiers, and show similarly equipped and armed fighters in different cities in eastern Ukraine.
There was no immediate response to the pictures from the Russian government.
Pro-Russian militants are holding official buildings in towns and cities in the east. However, Russia has denied it has any soldiers in Ukraine.
The photos, provided to the Organization for Security and Co-operation in Europe, appeared to show the same man taking part in operations in Kramatorsk and Sloviansk in eastern Ukraine, and in operations in Georgia in 2008.
Another group of photos appeared to link a gunman seen in photos in Kramatorsk and Sloviansk to a team in the Russian special forces.
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 Jen 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 destabilising 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.