Separatists have shot down a Ukrainian military plane in the eastern city of Luhansk, killing 49 people.
It is the biggest loss of life suffered by government forces in a single incident since Kiev began a military operation to try to defeat the pro-Russia insurgency in east Ukraine.
A statement released by the Defence Ministry accused pro-Russian "terrorists" of shooting down the Ilyushin-76 aircraft, which was about to land at Luhansk airport.
A separatist gathers ammunition at the site of the crashIt said the rebels "cynically and treacherously" downed the plane using anti-aircraft guns and heavy calibre guns at 11pm UK time on Friday (1am local time on Saturday).
The attack came as presidential bodyguards discovered a bomb, packed with five grenades and 1kg of metal nuts, at the gates of Petro Poroshenko's headquarters.
The device, which was discovered on Friday night and described as "really powerful", was disposed of by security services.
Everyone on board was killedThe plane was carrying nine crew and 40 paratroopers, as well as food and equipment.
The prosecutor's office said a criminal investigation had been opened under anti-terrorism laws.
Alexei Toporov, a defence spokesman for the self-proclaimed Luhansk People's Republic, said the aircraft was shot down after what he called Ukrainian "occupiers" had refused an ultimatum to leave Luhansk airport.
There were 40 paratroopers on the plane when it was downedFootage has been uploaded online, claiming to show the plane coming down.
In the CCTV footage, which Sky News has not yet been able to verify independently, a small flash of light is seen in the sky.
This is then followed by a second, much larger, burst of light which illuminates the horizon.
Luhansk is close to the Russian borderThe Luhansk Oblast is at the heart of the separatist rebellion launched in April, and local media is reporting that there is fighting in the region's administrative centre, also called Luhansk.
The rebels said the Ukrainian air force have fired on the industrial town of Horlivka, north of Donetsk.
It comes after Russia was accused by the US State Department of sending tanks and heavy weaponry to Ukraine in support of separatists in the country's east.
The first flash of light in the footageSeveral tanks and rocket launchers were moved into Ukraine from an arms depot inside Russia, the department said.
The statement comes after Kiev alleged that three tanks had crossed from Russia into Ukrainian territory this week.
Ukraine's President Petro Poroshenko told his Russian counterpart Vladimir Putin on Thursday that the action was "unacceptable".
A Russian tank seen at a military base near Simferopol in MarchState Department spokeswoman Marie Harf said three T-64 tanks, several BM-21 or Grad rocket launchers and other vehicles had crossed into Ukraine's east as part of a convoy.
Amateur videos posted on the internet in recent days show the same kind of tanks moving through cities in eastern Ukraine, including Snizhnye, Torez, and Makiyivka.
Moscow has denied the allegation.