The Pentagon says the movement of Russian heavy caliber artillery systems across the border into Ukraine is "imminent", escalating the military action there.

Col. Steve Warren, citing U.S. intelligence, says the U.S. has seen the systems moving closer to the border and they could be put into the hands of Russian-backed separatists as soon as Friday. He says he doesn't have an exact timeline.

Warren also says Russia continues to fire artillery across the border into Ukraine but the U.S. has seen no indications of Ukraine firing back into Russia. He says the number of Russian troops along the border continues to slowly but steadily increase. Close to 12,000 are there now.