In a major speech, Mr Poroshenko said Russia's Zapad-2017 exercises with Belarus might be "a smokescreen to create new Russian army assault groups to invade Ukrainian territory".
Russia says 13,000 troops will take part in Zapad, on 14-20 September.
It is possible that Ukraine is correct. Although I doubt if Belarus would be the jumping off point for an attack from the north because that would be a major provocation so close to NATO Poland.
If Russia needs the exercises to prepare troops for war, as mobilizatons were used prior to the 2014 aggression against Ukraine and prior to the 2008 war against Georgia, I'd suspect that troops in Belarus would be used to pin Ukraine's military in place while a strike is launched against Kharkov with an advance south to expand northward the eastern territory that Russia controls in the Donbas region; and to expand the existing Russian-occupied territory in the Donbas, perhaps with a limited effort to take Mariupol.
It doesn't seem wise for Russia to anger NATO and reinvigorate sanctions pressure and justify arms sales and closer cooperation with Ukraine. It doesn't seem wise to add to Russian casualties and risk the Russian people turning against the war and Putin. And it doesn't seem wise to strain Russia's finances with a wider war.
But with stalemate, perhaps the Russians believe the only way to win the war and end it is to escalate it beyond Ukraine's capacity to fight. I doubt Russia can manage that, but I could be wrong. Or the Russians could simply wrongly believe they have that capacity.
I certainly don't blame Ukraine for worrying about what Russia might do, given what the Russians have done already.
