Pro-Russian separatists shot down a military transport plane Saturday in the country's restive east, killing all 49 service personnel on board, Ukrainian officials said.
It was a bitter setback for Ukrainian forces, which have struggled to suppress an armed insurgency by foes of the new government.
Nine crew and 40 troops were aboard the Il-76 when it went down early Saturday as it approached the airport at the city of Luhansk, the Ukrainian prosecutor general's office said in a statement.
The plane's tail section lay with other pieces of scorched wreckage in a farm field near the village of Novohannivka, about 12 miles south of Luhansk. An AP reporter saw a dozen or more armed separatists inspecting the crash site.
The death toll exceeded the loss of 12 soldiers including a general on May 29 when rebels shot down a troop helicopter near the eastern city of Slovyansk.
The incident underlines questions about rebel access to military gear. Ukraine has accused Russia of permitting three tanks to cross the border where they were used by rebels. Russia denies supplying the separatists.
NATO released images on Saturday, said to show recent Russian tank movements near the border which "raise significant questions" on Russia's role.
The tanks seen in Ukraine, NATO said, "do not bear markings or camouflage paint like those used by the Ukrainian military. In fact, they do not have markings at all, which is reminiscent of tactics used by Russian elements that were involved in destabilizing Crimea."
Denis Pushilin, a leader of the separatist Donetsk People's Republic, told Russian state television Friday that rebels had the tanks but it was "improper to ask" where they got them.
The defense ministry's statement said that the rebels "cynically and treacherously" downed the transport plane using anti-aircraft guns and heavy machine guns. It expressed sympathy to the families of those killed "for their tragic and irreparable loss."
Alexei Toporov, defense spokesman for the self-proclaimed Luhansk People's Republic, said the aircraft was shot down after what he termed Ukrainian "occupiers" refused an ultimatum to abandon the Luhansk airport.
Luhansk is in Ukraine's east near the border with Russia, an area that has seen separatists seize government buildings and declare independence after holding disputed referendums.
Ukraine had claimed a success on Friday when troops retook some rebel-occupied buildings in the port city of Mariupol. No deaths were reported.
Before Saturday's incident, the Ukrainian health ministry had said at least 270 people had died in clashes between government forces and armed separatists.
Tensions between Ukraine and Russia escalated in February after pro-Russian President Viktor Yanukovych was driven from office by a massive protest movement made up of people who want closer ties with the European Union.
Another official, counter-terrorism spokesman Vladislav Seleznyov, was quoted by the Interfax news agency as saying the death toll was preliminary and was being verified by investigators.
The Russian-built Il-76 is a four-engine jet that can be used to transport heavy gear and people.
The prosecutor's office said a criminal investigation had been opened under anti-terrorism laws.
McHugh contributed from Kiev.