Hundreds of Russian troops highly trained in combat techniques are entering eastern Ukraine cities, the Kiev administration has said.
Despite Moscow's repeated promises to not involve itself in the crisis, the number of Russian soldiers on Ukrainian soil is steadily rising according to Vasyl Krutov, the newly-appointed chief of Ukraine's Anti-Terrorist Centre.
"Two days ago they [Russian soldiers] numbered 150, yesterday there were 300. I'm not including those that were here before that. These are the approximate numbers. Time will tell how many will be here tomorrow," Krutov told the UNIAN news agency.on Tuesday.
When asked how the Russian forces were able to penetrate Ukraine's borders, the Ukrainian official said: "For a professional to cross the border is the same for a child to sneak into a movie theatre."
Russia has officially said it will not militarily intervene in Ukraine. However, Ukraine and its western backers are expressing serious concerns over Moscow's increasing military presence on the borders.
Moscow has nonetheless warned the Kiev administration that there will be consequences if Ukraine uses force against pro-Russian protesters.
Ukraine on Brink of Civil War
Meanwhile, Russian President Vladimir Putin warned Ukraine was "on the brink of civil war" as he spoke with German Chancellor Angela Merkel by phone.
The Kremlin's official statement said: "Mr Putin and Ms Merkel exchanged views on the anti-constitutional course taken by the current authorities in Kiev to forcefully suppress protests in the southeast regions of Ukraine. The President of Russia noted that the sharp escalation of the conflict essentially puts the nation on the brink of civil war."
Despite Russia's stern warning, Ukrainian forces are said to be moving in on anti-Kiev activists in a bid to retake control of state buildings in several eastern Ukrainian cities.
The Ukrainian troops are launching a "special anti-terrorist operation" across the eastern cities. In Kramatorsk, a source told Russia's RIA Novosti at least four armed pro-Moscow protesters were killed in a shootout with Ukrainian troops.
"There are four dead and two wounded at the airfield. They are all militia. The fighting for the airfield is over as the militia has reiterated and the Ukrainian side took control of the airfield," added the source.