BRUSSELS - Nato released satellite pictures on Saturday that it said raised suspicions about Russia's role in moving military equipment into eastern Ukraine, where scores of people have been killed in fighting between government forces and pro-Moscow separatists.
Nato said the images showed Russian tanks arrived at a staging area close to the Ukraine border days before similar tanks appeared this week in areas of eastern Ukraine under separatist control, but where they came from is in dispute.
Ukrainian Interior Minister Arseny Avakov said the tanks had rolled across the border from Russia, but rebels said they were from a Ukrainian military stockpile.
The US State Department said on Friday its assessment was that separatists in eastern Ukraine had acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers.
The commercial satellite images released by Nato "raise significant questions concerning Russia's role in facilitating instability in eastern Ukraine and its involvement in the movement of military equipment from Russian territory into Ukraine," a Nato military officer said.
An image released by Nato dated May 30 shows a Russian unit was deployed near the southern Russian city of Rostov-on-Don as part of a military buildup close to the Ukraine border, but no tanks were present, the officer said.
Another satellite image from a week later, June 6, showed the Russian unit had left as part of a broader withdrawal from the Ukraine border area.
But while other vehicles had pulled out, the image showed eight main battle tanks had arrived, according to Nato.
In another image dated June 11, 10 main battle tanks can be seen at the site, three of them loaded on trucks, likely indicating imminent movement by road, the Nato officer said.
A day later, Ukraine accused Russia of allowing separatist rebels to bring three tanks and other military vehicles across the border into the east of the country.
Later that day, Reuters correspondents saw two tanks in the eastern Ukrainian town of Snizhnye.
The Nato military officer said the T-64 tanks seen in eastern Ukraine did not bear markings or camouflage paint like those used by the Ukrainian military. "In fact, they do not have markings at all, which is reminiscent of tactics used by Russian elements that were involved in destabilising Crimea," he said.
Russia's annexation of Ukraine's Crimea region and the uprising in eastern Ukraine have caused the worst East-West crisis since the Cold War.
Pro-Russian separatists shot down a Ukrainian army transport plane with an anti-aircraft missile as it came in to land early on Saturday in the eastern city of Luhansk, killing all 49 military personnel on board.
Nato Secretary-General Anders Fogh Rasmussen said on Friday that, if the tanks were confirmed to have come from Russia, it would mark a serious escalation of the situation in eastern Ukraine.
Russia still has approximately 2,000 T-64 main battle tanks which it has phased out of service and were slated for destruction, but it was highly likely that many of tanks were still operational, Nato said.
Following the ousting of Ukraine's pro-Russian President Viktor Yanukovich in February, Russia massed as many as 40,000 troops close to Ukraine's border, according to Nato, and Nato commanders voiced concern about a possible Russian military intervention.
Russia has now withdrawn most of its troops and Nato estimates that fewer than 1,000 remain close to the Ukraine border, the military officer said.