Ukraine says 13 of its servicemen have been killed and 20 injured in fighting with Russia-backed separatist forces over the past 24 hours.
A Security Council spokesman said the fiercest battles were still for control over Debaltseve, a government-held railway hub that the rebels need to further consolidate their control in eastern Ukraine.
The spokesman said the separatists were attacking several villages around Debaltseve.
More than a dozen Ukrainian tanks and other heavy military vehicles, including a rocket launcher, are reportedly heading towards Debaltseve, apparently to reinforce the government troops.
A month of relative quiet in eastern Ukraine was shattered in early January by full-blown fighting as the separatists attempted to claw back additional territory from the government.