(Dmitry Lovetsky/AP Photo)

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While that remains to be seen, in June, NATO's top general said that Russia had provided "anti-aircraft" training inside Russia for Russian separatists that involved "vehicle-borne" surface-to-air missiles.

At a June 30 Pentagon news conference, NATO Supreme Commander Gen. Philip Breedlove said Russia had been providing air defense training to Russian separatists on its side of the border with Ukraine that focused on "vehicle-borne" surface-to-air missiles. A vehicle-borne capability would involve a surface-to-air missile with a longer range than portable shoulder-fired missiles known as MANPADS.

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U.S. officials believe that Russian separatists in eastern Ukraine possess MANPADS, some of which may have been used in June to down a Ukrainian military aircraft and two helicopters.

"What we see in training on the east side of the border is big equipment, tanks, APCs, anti-aircraft capability, and now we see those capabilities being used on the west side of the border," Breedlove said at the time. He added that the anti-aircraft capability training focused on larger vehicle-borne missiles instead of portable MANPADS.

"We have not seen training of MANPADS," Breedlove told reporters. "But we have seen vehicle-borne capability being trained."

U.S. officials say an Antonov-26 cargo plane that crashed in eastern Ukraine on Monday was shot down by a surface-to-air missile.

On Wednesday, a senior administration official said the AN-26 aircraft "was shot down from an altitude of 21,000 feet, with eight crew on board. And only very sophisticated weapons systems would be able to reach this height."

Another U.S. official said that whether the missile had been fired from inside Russia had not been conclusively determined.

There has been a flow of heavy weaponry, including some tanks, that have flowed from Russia into eastern Ukraine. The official said the belief has been that Russian separatists did not possess larger surface-to-air missile systems aside from MANPADS.

There is still no determination on what may have led a Ukrainian SU-25 fighter jet to crash on Wednesday, though the Ukrainian military blames an anti-aircraft missile.

Breedlove also said at the late June Pentagon news conference that there was a "very good likelihood" that Russian air defense hardware may have been used to shoot down two Ukrainian helicopters that had crashed earlier in June.

"We don't know whether the first two shoot downs were MANPADS or vehicle-borne missiles. Again, I like to report when I have facts, so I will just tell you that we're not - we haven't tied that string completely together as to which was used in which situation."