It "strains credulity" that Russian separatists in eastern Ukraine did not get some Russian help in operating the sophisticated air defense missile system that brought down Malaysian Airlines Flight 17, the Pentagon said today.

But with or without assistance, a Pentagon spokesman cautioned that the U.S. still does not have firm information as to who fired the missile.

There is strong evidence that the SA-11 surface-to-air missiles was fired from territory in eastern Ukraine controlled by Russian separatists, but it's still unclear who fired the missile, Pentagon spokesman Rear Admiral John Kirby told reporters today.

(Dmitry Lovetsky/AP)

"Whether it was a Russian military unit that did it or it was a separatist unit that did it, we don't know," Kirby said. "Whether it was a system that was driven across the border by Russians and then handed off, we don't know. We just don't know."

However, Kirby said it "strains credulity" that such a sophisticated piece of technology "could be used by separatists without at least some measure of Russian support and technical assistance."

On June 30 by NATO Supreme Commander Gen. Philip Breedlove that Russia had provided training for "vehicle-borne" air defense systems to Russian separatists.

Kirby said today "there's no question" that kind of training had occurred inside Russia, though it remained unclear who was doing the training.

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He said the U.S. had taken that training seriously and referred to a Notice to Airmen (NOTAM) cautioning commercial flights over Ukraine to fly at higher altitudes. He did not know if the NOTAM was prompted by the training in vehicle-borne air defense systems, but said "if you're going to issue a warning like that, it's based on concerns that you have about surface-to-air missile activity and capabilities."

The U.S. has detected heavy Russian weaponry, including tanks and armored personnel carriers, being shipped across the border into Ukraine for use by the separatists. But the Pentagon's top spokesman said he had no indications to suggest that an SA-11 system had been brought into Ukraine from Russia though "we're not ruling anything out at this point."

Kirby cautioned that "we don't have perfect visibility into every capability that the separatists have," but he noted that the U.S. believed the Russian separatists aspired to have access to an air defense capability - both portable and vehicle-borne.

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Russian separatists used shoulder-fired surface-to-air missiles to bring down two helicopters and a plane in June and may have used a bigger air defense system to bring down a Ukrainian military transport plane on Monday.

There continue to be 10,000 to 12,000 Russian troops along the border with Ukraine, Kirby said, noting that the size of that force has steadily increased over time. Their presence only serves to escalate tensions and "it's difficult to know what the intent is," he said.

There are no plans for a Pentagon representative to be a part of the U.S. team that will help in the international investigation in Ukraine, Kirby said.

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