Tom Vanden Brook

USA TODAY

The Russian military fired artillery rounds into eastern Ukraine on Thursday, a "clear escalation" of hostilities there, Army Col. Steve Warren, a Pentagon spokesman, said.

Russia has about 12,000 troops on the border with Ukraine, Warren said. Russian tanks, artillery and rocket launchers have crossed into Ukraine to support separatists there.

Defense Secretary Chuck Hagel has not spoken to his Russian counterpart about the artillery attack and has no plans to, Warren said.

"This is a clear escalation," Warren said.

The Russians have conducted artillery attacks on Ukraine military targets for the last several days, Warren said.

The State Department also said Thursday they had evidence of the Russian attack.

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

The allegations come a day after two Ukrainian attack planes were shot down over rebel-held territory. Ukraine's government said the missiles were fired from Russia. U.S. defense and intelligence officials said they could not verify the claim.

Contributing: Jim Michaels