Russia's President Vladimir Putin warned Ukraine was "on the verge of civil war" in a phone call to German Chancellor Angela Merkel, after acting President Olexander Turchynov announced the start of the operation to reclaim territory.
Russian officials said both leaders "emphasised the importance" during the conversation of planned four-way talks on Thursday between senior diplomats from Russia, the EU, the US and Ukraine.
However, Ukrainian and Western officials have accused Russia of being behind the pro-Russia activism in the region. Moscow strongly denies fomenting the unrest.
Ukrainian counter-terrorism chief Gen Vasyl Krutov, speaking to reporters at Kramatorsk airfield late on Tuesday, emphasised that armed individuals had crossed the border into Ukraine.
"We have to deal with a very serious, highly skilled and very professional opponent," he said according to Interfax-Ukraine news agency.
"They display a very high level of preparedness, tactical and practical skill. These people must have been to hot spots across the world and on their own territory."
Last month's annexation of Crimea by Moscow was preceded by the presence of uniformed, armed men believed to be Russian troops, although in greater numbers than the similarly dressed and equipped men who have been seen in eastern Ukraine, correspondents say.
The Kremlin has condemned the Ukrainian military operation in eastern Ukraine as an "anti-constitutional course to use force against peaceful protest actions".
16 April 2014
Now that Ukraine is taking actions against pro-Russian violence-mongers, is a true civil war about to break out?
Posted by Brant at 12:18