At least 15 troops and 50 pro-Russian militants have been killed since Monday as Ukrainian forces engaged in fierce fighting against militias in the country's eastern regions of Donetsk and Lugansk.
A military official confirmed Tuesday the casualties suffered on the government troops' side.
Forty-nine other Ukrainian servicemen were wounded, Xinhua quoted Andrei Lysenko, spokesman for Ukraine's National Security and Defence Council, as saying.
Earlier in the day, a military command said government forces killed 50 pro-Russian militants and destroyed a Grad missile launcher and two armoured vehicles.
"Terrorists who wear the uniform of the Russian army" attacked Ukrainian units carrying out medical evacuation of the wounded, EFE quoted the command as alleging.
Meanwhile, Lysenko repeated Kiev's claim that Russian forces are helping the insurgents, saying the "movements of Russian troops" were reported in six towns in the Donetsk region and in four communities in the Lugansk region.
The official also said shells were fired from Russian territory against Ukrainian troops stationed near the joint border.
Ukrainian forces are battling between 10,000 and 15,000 armed opponents, including militia and regular Russian troops, he added.
The pro-Russian militias recently launched a major counteroffensive that forced the Ukrainian troops to retreat from several strategic points, including the Lugansk international airport.
Over 2,500 people have been killed and at least 400,000 others have fled the restive Donetsk and Lugansk regions since mid-April.
