NATO Secretary General Jens Stoltenberg expressed concern about what he called a "significant military build-up".  "This is a severe threat to the ceasefire," he said. "I call on Russia to pull back its forces and equipment from Ukraine, and to fully respect the Minsk agreements."  Read MoreJohn Kerry: US, Russia to exchange info on Ukraine crisis  President Vladimir Putin has accused the West of instigating a coup that ousted a Moscow-backed president in Kiev in February after months of street protests, and of trying to use the crisis to prevent Russia's rise as a global power.  Russian Foreign Minister Sergei Lavrov told U.S. Secretary of State John Kerry by phone that the ceasefire deal must be upheld, rejecting accusations that Moscow is to blame for its collapse.  A Reuters reporter, however, saw unidentified military trucks in the center of Donetsk on Wednesday, with soldiers in green uniform without insignia standing nearby. Russian soldiers spotted by local residents have often worn no insignia.  Kiev's fear is that Putin, who annexed the Crimean peninsula from Ukraine in March, wants to expand the territory controlled by the separatists with another military push like the one that turned the tide in the rebels' favor in August.  Read MoreTank column crosses from Russia into Ukraine: Kiev military  Moscow denied sending in troops and armor in August but said some Russians may have been there as volunteers during their holidays. A large number of Russian soldiers are among the dead in the conflict.  The prospect of all-out war returning to eastern Ukraine has piled pressure on the country's struggling economy, sending the hryvnia currency plummeting. The cost of insuring exposure to Ukraine's debt hit five-year highs on Wednesday, while its dollar bonds were sold off heavily.  Russia is also suffering an economic downturn aggravated by Western economic sanctions over the conflict, with the rouble falling nearly 30 percent against the dollar this year.  But Putin has shown no sign of changing policy on Ukraine and the EU has signaled it will not ease sanctions on Russia when it meets to discuss them next week.
Ukraine said on Wednesday it was redeploying troops in the east because of fears that separatists will launch a new military offensive.