Russian military forces have fired artillery rounds into eastern Ukraine targeting Ukrainian military positions and plans to send even more heavy multiple rocket launchers to Russian separatists, U.S. officials said for the first time today.

A Pentagon official labeled the Russian activity as "a clear escalation" of the conflict in that region that has drawn worldwide attention since Malaysia Airlines Flight 17 was shot down.

"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine, and have evidence that Russia is firing artillery from within Russia to attack Ukrainian military positions," said Marie Harf, a State Department spokeswoman.

Harf did not provide additional information as to how the U.S. concluded that the Russian military had been shelling Ukrainian military positions. However, she said that the information that the heavier rocket launchers were headed into Ukraine had been gleaned from what she referred to as "human intelligence."

Pentagon spokesman Col. Steve Warren later said, "We do know now that the Russians have been firing artillery from Russia into Ukraine to attack Ukrainian military positions."

"This has been happening, we believe, for several days," said Warren. "This is a military escalation, there's no question about it."

As evidence that the Russians intend to deliver heavier multiple rocket launchers to separatist forces in Ukraine, Warren referred to activity at a site in southwestern Russia where the Russian military has provided training for Russian separatists and gathered heavy military equipment for their use.

Senior intelligence officials referred to the site outside of Rostov on Tuesday when they made their case for why Russia had "created the conditions" for the shoot-down of the Malaysian airliner with a Russian-built SA-11 surface-to-air missile.

Since the shoot-down "there's been ongoing multiple rocket launcher activity at the Rostov site and multiple rocket launchers continue to depart and return to Rostov at irregular intervals," Warren said.

For the last month the Russian military has gathered between 10,000 and 12,000 troops along the border with Ukraine, officials said. The forces are arrayed along the border at varying distances, with some elements coming as close as five miles to the border with Ukraine, one official said.

It has been difficult to determine what the Russian intent is for these forces, the official said, but the information released today indicates at least some of them have been engaged in artillery fire targeting Ukrainian military positions.

Ukrainian officials have alleged direct Russian military support for separatists for weeks. Over the past 24 hours pictures have appeared in Ukrainian media outlets that were purportedly taken by a Russian soldier displaying the artillery equipment his unit has used during its deployment to the border with Ukraine. The images have since been taken down from the Russian social media site they were originally posted.

While officials did not provide a precise time as to when the Russian military began to fire artillery across the border, a U.S. official said it began as early as July 15.

Videos posted on social media on July 16 appeared to show Grad rockets being fired from inside Russia into Ukraine. The Grad system is a multiple rocket launcher system used by the Russian military that has been provided to Russian separatists.

NATO Supreme Commander Gen. Philip Breedlove tweeted his concerns about the videos the following day. "I am deeply concerned by this latest video that appears to show Russia engaging in military action against Ukraine," he wrote.

Senior intelligence officials said Tuesday that the flow of Russian heavy military equipment has continued into eastern Ukraine since the crash of the Malaysian airliner. The flow of weapons included tanks, armored personnel carriers and Grad multiple rocket launcher systems.