Ukraine said on Wednesday it was redeploying troops in the east because of fears that separatists will launch a new military offensive, despite Russia's denials it has sent troops to reinforce the rebels.
A ceasefire agreed by the pro-Russian rebels and government forces more than two months ago is now all but dead, and Western fears of a return to all-out conflict are growing.
U.S. General Philip Breedlove, NATO Supreme Allied Commander Europe, said the alliance had seen Russian troops and tanks entering Ukraine in the past few days, confirming reports by international observers.
"There is no question any more about Russia's direct military involvement in Ukraine," Breedlove said in Bulgaria.
Read MoreRussian troops enter Ukraine: NATO commander
A Russian Defense Ministry official, General-Major Igor Konashenkov, said in Moscow that "there were and are no facts" behind such statements and Russia had given up paying attention to such accusations by NATO.
Ukrainian Defense Minister Stepan Poltorak left no doubt that Kiev was also no longer paying attention to Moscow's denials of providing the rebels with direct military support in the worst diplomatic standoff with the West since the Cold War.
"We are repositioning our armed forces to respond to the actions of the (rebel) fighters," Poltorak told a government meeting in Kiev. "My main task is to prepare for military action."
He gave no details of the troop movements.
The ceasefire was agreed in the Belarussian capital of Minsk on Sept. 5 after weeks of fierce fighting between government forces and separatists who rebelled in mainly Russian-speaking eastern Ukraine against the rule of Kiev's Western-looking government eight months ago.
The truce has been violated daily, and increasingly since the rebels held what the West and Ukrainian President Petro Poroshenko said were illegitimate leadership elections on Nov. 2. The death toll has passed 4,000 since the truce was agreed, with Kiev accusing Moscow of sending more troops last week.