The US Department of State (DoS) has released a series of photographs of unmasked men, which link to Russian special forces, affirming the country's involvement in stirring up unrest in eastern Ukraine.
Provided by the Ukrainian government to the Organization for Security and Co-operation in Europe (OSCE), the images show Russian soldiers in operations in Georgia in 2008, and similarly equipped and armed rebel activists in the Ukrainian cities of Kramatorsk and Sloviansk.
Another set of photos, which show two rebels occupying state buildings in eastern Ukrainian cities, is being linked to the same men present in a pair of 'family photos', which Ukraine claims are of the Russian special forces' 'sabotage and reconnaissance' group.
US State Department spokesperson Jen Psaki said: "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 US officials have continued to make that case."
However, OSCE eastern-Ukraine monitoring mission head Klaus Zillikens was quoted by the Ekho Moskvy radio station as saying that there is no evidence that Russian soldiers are involved in the seizure of state buildings in Donetsk.
"There are signs of foreign consultants working in the Ukrainian territory, but no clear proof of that," Zillikens said.
Russian officials, including President Vladimir Putin have already denied the presence of Russian soldiers.
Putin said: "There are no Russian units, special services or instructors in the east of Ukraine."
Pro-Russian activists are occupying official buildings in at least ten eastern Ukrainian cities, and have demanded greater autonomy from the new Ukrainian government and closer ties with Russia.