Ukrainian military losses
It's hard to tell the exact number of Ukrainian military casualties, as Ukraine has not revealed official data on those that were killed in the Donbas war to journalists and the public. There are numbers that are released by Ukraine's Ministry of Defense on servicemen killed in action. However, not all those that are killed are accounted for in those reports, as they include only battalions that are subordinated to either the Ministry of Internal Affairs or the Ministry of Defense. The volunteer battalions that are fighting along the regular Ukrainian Army in Donbas have had a complicated history of becoming part of either the MIA or MoD, and it is unclear whether losses from early days are included in the statistics. There are some volunteer battalions that are still not subordinated to either of those Ministries (the Right Sector and OUN battalions that are part of the Ukrainian Volunteer Corps). They and other losses from border patrol guards, the Security Service of Ukraine, and unidentified losses have been taken into account in an article on Wikipedia that is kept updated by volunteers. Oleksandr Oksymets from Ukrainian website texty.ua has calculated losses based on this independent Wikipedia bookkeeping. According to him, 1521 military servicemen have died in the Donbas war from its start in March 2014. 411 of them have died during the ceasefire that was announced following the signing of a peace protocol in Minsk on September 5 between the separatist leaders and representatives of the Ukrainian government. The majority of those that died, 1136, were subordinated to the Ministry of Defense; the National Guard and battalions under the Ministry of Internal Affairs suffered 218 losses (click to enlarge).
However, it may very well be that the actual military losses are much higher. It is unclear to this day how many casualties were carried in the most tragic battle for Ukraine, the battle of Ilovaisk, during which surrounded Ukrainian troops came under enemy fire when retreating through a "green corridor." The accounts vary significantly. Donbas battalion commander Semen Semenchenko claims over 1000 troops were killed in Ilovaisk; this number is supported by a temporary investigative committee. However, only 241 deaths have been officially recognized by the Prosecutor's Office as of 11 December. Azov battalion commander Andriy Biletskyi is also convinced that at least 1000 lives were taken in the tragic battle, which is being investigated at present to find those guilty of such losses. Thus, if 1000 perished in Ilovaisk and not 241, the Ukrainian military losses could reach over 2000.