WASHINGTON -- The movement of Russian heavy-calibre artillery systems across the border into Ukraine is "imminent," the Pentagon said Friday, saying Russia is escalating the military action there.

A Pentagon spokesman, Col. Steve Warren, said the U.S. has seen the powerful rocket systems moving closer to the Ukraine border and they could be put into the hands of Russian-backed separatists as soon as Friday.

U.S. officials warned this week that they had new evidence that Russia intended to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine. Warren told reporters that the delivery could happen at any time.

Warren also said Russia continues to fire artillery across the border into Ukraine.

"For the last several days, Russian forces using Russian artillery from Russian soil have conducted attacks against Ukrainian military positions in Ukraine," Warren said. "This is unquestionably an escalation from a military perspective."

He declined to say what impact the attacks have had on Ukraine's military, but said the attacks are coming from the southern border area, near Rostov-on-Don.

Warren said the U.S. has seen no indication of Ukraine firing back into Russia, and there have been no reports of civilian casualties. He said the number of Russian troops along the border continues to slowly but steadily increase. Close to 12,000 are there now.

The Ukrainian army on Friday said soldiers had come under artillery fire from the Russian side of the border overnight and were attacked by rebels in several other places in the restive east.