Three Ukrainian servicemen were killed and 15 wounded during fighting with pro-Russian rebels in the country's east in the past 24 hours, a spokesman for Ukraine's military operation said Tuesday.
Another soldier went missing in action, spokesman Olexandr Motuzyanyk said in comments carried by state news agency Ukrinform.
A separatist group in Luhansk told Russian state news agency TASS that the Ukrainian military attacked rebel positions twice over the past 24 hours. Rebels in the neighbouring Donetsk region also reported fighting.
Tensions between Russia and Ukraine have escalated in recent weeks, with both countries putting troops on full combat alert after Russia accused Ukrainian forces of planning attacks in the Crimea region.
Russia occupied and annexed Crimea from Ukraine two years ago in response to Kiev ousting its pro-Russian president amid mass protests calling for closer ties with the West.
Russia has since supported the simmering separatist rebellion in Ukraine's two easternmost regions, but has denied allegations that it has provided weapons and troops for the revolt.
More than 9,000 people have been killed in the conflict, according to estimates by the United Nations.