Rebels shot down a cargo plane over the airport of the eastern city of Luhansk last night killing 49 Ukrainian military personnel.
Luhansk is one of the centres of a pro-Russian separatist movement in Ukraine's east which has risen up against the pro-European government in Kiev.
A military spokesman, Vladislav Seleznyov, later said that 49 had been killed.
The toll is the biggest suffered by government forces in a single incident since Kiev started a military operation to try to defeat the insurgency in east Ukraine, which the rebels want to be part of Russia.
Ukraine's new Western-backed President Petro Poroshenko vowed to deliver an "adequate response" to the rebels.
Scores of people, including rebels, civilians and government troops, have been killed in the violence since April.
The US secretary of state John Kerry said that the US and G7 will raise costs for Russia if it does not end the weapons flow across the border.
Mr Kerry warned Russia that the US and its G7 partners would "raise the costs" Moscow could face unless it curbed weapons flowing into Ukraine and cut ties with pro-Russia separatists.
He also expressed condolences for the deaths of 49 servicemen who were killed when pro-Russian separatists shot down an army transport plane in east Ukraine.