June 15 declared day of mourning for Il-76 crash victims in Ukraine
"I declare Sunday a day of mourning for our military. It's a big loss not only to the victims' families, but also to the entire country. Ukraine grieves, but we continue fighting for peace," Poroshenko was quoted by the presidential press service as saying.
The president has extended his condolences to the families of the military and border guards who were killed on June 14.
"Everyone involved in this cynical terrorist attack of such scale will definitely be punished. Ukraine needs peace. However, terrorists will receive an appropriate response," he said.
Poroshenko ordered that a meeting of the National Security and Defense Council be held and asked the Cabinet of Ministers to help the victims' families.
According to earlier reports, a Ukrainian military-transport Il-76 plane was shot down by a surface-to-air missile system while landing on the Lugansk aerodrome in the early hours of Saturday.
The plane was carrying forty Ukrainian troops and nine crewmembers. All of them were killed, the Ukrainian Prosecutor General's Office reported on its website.