Pro-Russian separatists have shot down a Ukrainian military transport plane 10km from the eastern city of Luhansk, killing 49 people on board, a defence spokesman has said.
Ukrainian officials offered condolences on Saturday to the families of the killed soldiers.
A ministry statement said "terrorists" had "cynically" fired a large-calibre machine gun hitting a Ukrainian air force Ilyushin-76 as it was about to land at Luhansk airport, the AFP news agency reported.
Al Jazeera's Kim Vinnell, reporting from Donetsk, said the Ukrainian military had launched air strikes on separatist positions inside Luhansk in retaliation for the downing of the plane.
Close to the Russian border, Luhansk is the main city of one of two eastern regions hit by violence and which has declared its independence from Kiev.
The airport is the only area of Luhansk still under Ukrainian government control, Vinnell said.
On Friday morning, Ukrainian troops, including special forces, launched an operation to expel pro-Moscow fighters in the port city of Mariupol, leaving five rebels and two soldiers dead.
Ukrainian Interior Minister Arsen Avakov called the raid a success, and witnesses said that at least four separatists had been arrested, the AP news agency reported.
Ukraine and the West accuse Russia of supplying separatists with weapons, supplies and fighters.
Also on Friday, the US confirmed earlier reports that a convoy of armoured vehicles, including three Russian T-64 tanks, had moved in to Ukraine from Russia and were now in rebel hands.
US State Department spokeswoman Marie Harf said Washington had information that Russia had accumulated tanks of a type no longer used by its own forces in southwest Russia, and some had recently left the area.
Harf said the alleged transfer of arms from Russia to rebels in Ukraine was "unacceptable".
An "anti-terrorist" clampdown by Ukrainian armed forces has left at least 270 people dead in eastern Ukraine over the past two months.