Military spokesman Vladimir Polevoi said Sunday at least 13 soldiers were killed and 20 others injured in fighting with pro-Russian separatists in the past day. Rebel leaders said seven civilians had died and at least 20 injured in separatist-held territory.

According to Polevoi, the fiercest battles were concentrated around the government-held town of Debaltseve, a major railway hub connecting the two main rebel strongholds of Donetsk and Luhansk.

"There is no question of encirclement or cutting off of the main communication lines...the situation is under control," the spokesman said, adding however that several nearby Kyiv-held towns had come under rebel attack.

Violence in the east of the country died down late last year, but flared again in January after the rebels announced a new offensive to claw back territory from Ukrainian government forces.

The renewed fighting has sent hundreds of Debaltseve's 25,000 residents fleeing their besieged town, which has been without power and water for the past 10 days. Many of those who have chosen to stay behind have sought refuge in underground shelters.

The United Nations says more than 5,100 people have died, and 900,000 have been displaced, since the conflict Ukraine's east broke out last April.

Peace plan 'under threat'

Hopes for de-escalation in eastern Ukraine appeared to evaporate on Saturday, when peace talks between separatist envoys, Ukraine, and Russia in Minsk failed to make any progress and broke up after just a few hours. Saturday's talks in the Belarusian capital came after an attempt on Friday failed. There has been no mention of whether further talks will take place.

The Organization for Security and Co-operation in Europe (OSCE) - which is part of the contact group formed to find a solution to the Ukraine conflict - on Sunday blamed the rebels for the failure. In a statement, it said the separatist envoys were not ready to talk about the key points of a peace plan, including a ceasefire agreement signed at a previous round of talks in Minsk last September.

The representatives "were not even prepared to discuss implementation of a ceasefire and withdrawal of heavy weapons," the OSCE said, adding that the rebels had instead pushed for the truce to be totally revised.

"Unfortunately the peace process is now under threat," Valeriy Chaly, the deputy head of Ukraine's presidential administration wrote on his Facebook page.

US Secretary of State John Kerry is expected to fly to Kyiv on Thursday to meet with Ukraine's leadership.

nm/bk (Reuters, AFP, AP, dpa)