"The insurgents planned to stage terrorist acts in Kiev and Kharkiv to destabilise the situation in the country," a spokeswoman for the SBU security service, Olena Gitlyanska, said on Facebook.
A special forces officer and a member of the insurgent group, who opened fire, were killed in operations in those cities late yesterday and early today, she said.
Another special forces officer was wounded but his condition was stable, she said.
"The Russian nationals were detained in another apartment. Twenty three more explosive devices were found there," he said.
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Gitlyanska added officials had also found automatic weapons, more than 40 grenades and falsified documents.
Tkachuk did not rule out that the detained Russian nationals were linked to Russian special services.
Kiev and the West have accused Russia of fuelling an insurgency in eastern Ukraine and sending in regular troops across the border in a conflict that has claimed more than 9,000 people since April, 2014.