Ukraine is observing large-scale military drills in the southern Mykolaiv region under the watchful eye of President Petro Poroshenko.
The drills come as Ukraine's military says two servicemen were killed and seven others wounded in separate rebel attacks over the weekend.
The latest violence has revived concerns that a fragile ceasefire between Kiev forces and pro-Russian separatists could collapse.
Poroshenko wants to reverse Russia's annexation of Ukraine's Crimea region last year, but insists Ukraine will not be the first to fire and break a peace agreement brokered in Minsk.
"I hope We will soon see the Ukrainian territories liberated, but I want to once again stress, that the Ukrainian Armed Forces will not be the first who start an offensive. Ukraine is strictly following its obligations within the Minsk agreement," says Poroshenko.
But his Russian counterpart Vladimir Putin says Moscow's seizure of Crimea righted a historical injustice, according to local news reports on Sunday.
Meanwhile, in southwestern Russia, similar military drills are also underway, with soldiers firing blank shells.
Both the Ukrainian military and the rebels accuse each other of violating the peace agreement, in a conflict that has killed more than 6,000 people so far.