"Back in August, we saw a spike in direct Russian intervention in the form of Russian military units. We saw Russian battalions as coherent formations deployed into Ukraine," Lute told a news briefing before a NATO defence ministers' meeting on Thursday, Censor.NET reports citing Reuters.
"We don't see another such direct intervention (by Russian units) as imminent," he said.
However, he said that Russian intelligence operatives and soldiers, in and out of uniform, had been serving in southeastern Ukraine since the beginning of the armed rebellion some nine months ago, mainly performing a command and control role.
"They have a sort of parallel command structure that shadows the separatists' command structure in an effort to control what is going on the ground," Lute said.
"They are only partly successful with this. We don't think that they have absolute control over the separatist movements, but we think they are trying ...
"There are Russian soldiers in southeastern Ukraine today operating the more sophisticated Russian hardware that has been provided to the separatist movement." he said.
Lute said it took the United States years to train crews on similar advanced equipment, which included electronic warfare equipment and more sophisticated command and control and air defence systems:
"This is not equipment that is subject to be handed over to a group of separatists with very sketchy military backgrounds and operated effectively."