WASHINGTON, D.C. -- Russian President Vladimir Putin appears to be doubling down on Ukraine, stepping up Russia's role in the fight between Ukraine's government and the rebels.

The U.S. and Ukraine are accusing Russia of firing artillery on Ukrainian troops from its Russian soil and preparing to move more heavy weapons over the border.

This marks a dangerous escalation in the most serious tensions between Russia and the west since the Cold War ended.

Screen shot of video believed to show rockets from Russia being fired into Ukraine. CBS News

Not every video can be verified, but U.S. officials say that half a dozen times over the past two weeks Russia has fired barrages from its own territory into Ukraine. The shelling, which Russian soldiers have been bragging about on the Internet, lasts about a half an hour and appears to be aimed at Ukrainian military units fighting the separatist rebels.

Chairman of the Joint Chiefs of Staff General Martin Dempsey said Putin is showing no sign of being cowed by the international condemnation of Russia's alleged role in the shootdown of the Malaysian airliner.

"There may be some folks who could convince themselves that Putin would be looking for a reason to deescalate. He's actually taken a decision to escalate," Dempsey said Thursday.

The shelling began at the same time Russia increased the flow of weapons to the rebels in Ukraine, including tanks, artillery, multiple rocket launchers and apparently the surface-to-air missile which shot down the airliner.

The weapons are identical to those in the arsenals of the Ukrainian military, allowing the rebels to claim they captured them on the battlefield.

They are coming from a base in southern Russia, where a much larger caliber rocket launcher has been spotted in recent days. A comparison of satellite photos from June and July show how much the supply base has expanded in the past month.

Satellite photos from June and July show how much the supply base has expanded.

Both the shelling and the arms shipments appear to be driven by the fact that the rebels have suffered a series of setbacks at the hands of the Ukrainian military.