Putin biding his time? Evidence shows buildup of Russian tanks, generals in Ukraine

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New evidence is exposing Russia's deepening role in the conflict in eastern Ukraine, suggesting Vladimir Putin may be preparing to gobble up more territory undeterred by NATO -- as a new U.S. military document highlights the extent of Moscow's aggression.

Bloomberg View reported that Ukraine security officials have found five Russian generals are playing a lead role commanding separatist units inside the country, despite Kremlin denials. This comes as aerial video shot by a drone -- and provided by pro-Ukraine fighting units -- depicts the build-up of a Russian military base with tanks and other gear in eastern Ukraine.

"We see a more assertive Russia trying to intimidate neighbors and change borders," NATO Secretary General Jens Stoltenberg recently said.

The NATO secretary general spoke after inspecting one of six new forward bases the alliance is building in Romania and the neighboring Baltics -- a response to Russia's actions in Ukraine. It's the biggest reinforcement of NATO's collective defense since the end of the Cold War.

Whether it will be enough remains to be seen. Retired Maj. Gen. Bob Scales, a Fox News military analyst, said that western action may be no more than a "pinprick" at this stage and that Putin's got five or six "modernized divisions" on the Ukrainian border.

Meanwhile, the U.S. military released an updated "National Military Strategy" which mentioned Russia seven times -- even more than it cited the Islamic State threat.

The report said while Russia has helped in certain counterterrorism and counternarcotics efforts, it has "repeatedly demonstrated that it does not respect the sovereignty of its neighbors and it is willing to use force to achieve its goals."

The strategy said: "Russia's military actions are undermining regional security directly and through proxy forces." In a startling passage, the document also said, "Today, the probability of U.S. involvement in interstate war with a major power is assessed to be low but growing."

The Kremlin responded angrily Thursday to the new U.S. military strategy, calling it "confrontational."

In a sobering assessment, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said since the last such military strategy was published four years ago, "global disorder has trended upward while some of our comparative advantages have begun to erode."

The latest intelligence showing five Russian generals are leading Russian-backed separatists in Ukraine reportedly was presented in an intelligence dossier to the White House and Congress last month.

According to Bloomberg, the document goes beyond past accusations by naming specific Russian generals.

The drone footage of the military camp, meanwhile, was first reported by The Daily Beast. The report said the footage initially showed a few tents and vehicles. The drone flew over the same area two weeks later, exposing a full base with tanks and new roads.

According to the report, the base is located between one Ukrainian-held area and one separatist-held area -- and the camp could be part of any upcoming offensive against the vital city of Mariupol.

Gen. Philip Breedlove, the Supreme Allied commander in Europe, estimates 7,000 Russian troops are currently in Ukraine, despite Russian denials.

"What smart people are saying is that Mr. Putin is very clear that he does not want Kiev leaning to the West and he will use the appropriate force necessary to keep Kiev from leaning to the West," he said at a June 25 news conference in Brussels

"Putin's got lots of time to do this, he's in no hurry," Scales said. "He's waiting for the opportunity to be about right and then the Russians are going to continue their advance."

Fox News' Jennifer Griffin and Lucas Tomlinson contributed to this report.