Author analyzed event and concludes it's a unit Спецназ (Alpha) СБУ - SWAT (Alpha) SBU.  History of the Spetsnaz units in Ukraine after collapse of the USSR - here.

There is a US Security Guard located in Kyiv, Ukraine. Training exercises here and TopGuard website here.

In early RT publication, the propaganda line of Blackwater was issued. In updates, the mercenaries were dubbed being from Greystone Ltd.

"It was reported that the tough guys are employees of Greystone Limited, a subsidiary of Vehicle Services Company LLC belonging to Blackwater/XE/Academi."

Who is Greystone?

US mercenaries deployed in Southern Ukraine

Although NATO has officially stated it would not intervene in Crimea, according to Russian political scientist Alexander Dugin, Greystone Limited, a subsidiary of Academi (formerly Blackwater), has kicked off its deployment in Ukraine.

Mercenaries arrive in groups - dressed in civilian clothes and carrying bulky packs - at Kiev airport, from where they are apparently sent to Odessa, in the southern part of the country.