Ukraine’s President Petro Poroshenko has vowed to retaliate after pro-Russia separatists shot down a military plane in the east, killing 49 people.
Those involved in such “cynical acts of terrorism” must be punished,” said Mr Poroshenko – who summoned security officials for an emergency meeting.
The defence ministry said the transport plane came under anti-aircraft fire over the city of Luhansk overnight.
It was about to land there, carrying troops and military equipment.
It is thought to be the biggest loss of life suffered by government forces in a single incident since the Ukrainian government in Kiev began an operation to try to defeat the insurgency in the east.
“The terrorists cynically and treacherously fired with a large-calibre machinegun, hitting an Ilyushin-76 of the Ukrainian air force,” the defence ministry said.
Footage posted on YouTube claims to show the burning wreckage of the Ukrainian military plane In a statement released later on Saturday, Mr Poroshenko said: “Ukraine needs peace. However, the terrorists will receive an adequate response.”
Analysis: David Stern, BBC News, Kiev
The question now is how the government will respond. After the shock of the large number of deaths – the biggest single loss suffered by the military so far – pressure will mount for newly-elected President Poroshenko to end the conflict.
But will it mean an added impetus to reach a ceasefire, or a further escalation of the fighting? And what of the arms used to bring down the large transport plane – were they Russian-supplied, as Ukrainian officials have insisted?
The chances for peace in Ukraine’s east still exist. But with every major incident like this one, they seem to recede even further into the distance.
The incident came less than a week after pro-Russia rebels launched a series of attacks on Ukrainian forces at Luhansk International Airport.
The airport has been under the control of government forces but the rebels hold most of the rest of the city.
Luhansk is the main city of one of two eastern regions where pro-Russia separatists have declared independence from Kiev.
The “anti-terrorist operation” by Ukrainian government forces has left at least 270 people dead over the past two months.
Mr Poroshenko had said that fighting must end “this week” following his inauguration in Kiev last Sunday. (BBC)