"Russia has sent a number of armoured personnel vehicles, tanks and troops. We can confirm that Russian military boots are on Ukrainian ground," according to Yatseniuk.
A column of armoured vehicles was spotted near the Russian side of the border. While it isn't clear where they had come from or where they were headed, Kiev says the latest reports suggest troops are working with separatists to stage an offensive in smaller cities east of Donetsk where fighting has been raging since April. Much of the city has become a war zone.
The latest developments were enough to prompt President Petro Poroshenko to cancel a planned trip to Turkey.
And in Kiev, Yatseniuk is asking the west for more help: "We urge the EU and the United States to freeze all Russian assets until Russia pulls back its forces, its military and stops the supply of Russian-led guerrillas."
But those guerrillas say they're digging in. Despite a death toll that exceeds 2,200, they say they'll fight to keep the territory they now hold.