Ukraine's President Petro Poroshenko warned on May 6 of an increasing threat of extensive military action in the east of the country.
Poroshenko told a security and defense council meeting in Kyiv that "the threat of large-scale military action from Russian terrorist groups not only remains but is growing."
According to a statement posted on his website, Poroshenko said there were more than 40,000 pro-Moscow fighters in the rebel-controlled territories and a further 50,000 Russian troops across the border.
Military spokesman Andriy Lysenko said four Ukrainian servicemen had been killed by a landmine and a fifth in a rebel ambush since May 5.
Casualties are reported on a regular basis despite the truce, brokered in February in the Belarusian capital Minsk to end a year-old conflict in which more than 6,000 have been killed.
Based on reporting by Reuters and Interfax