Ukraine's security service said on Thursday it had broken up an armed insurgent group that operated out of Kyiv and other major Ukrainian cities and that two people had been killed during the operation.
The SBU security service said it had detained three Russian and four Ukrainian citizens after a shootout in which a special forces officer and the Ukrainian leader of the armed group were killed.
"Just in one place in the capital we found eight homemade explosive devices, four kilograms of TNT, automatic weapons, about forty hand grenades and 2,000 cartridges," spokeswoman Olena Gitlyanska said in a Facebook post.
Kyiv has repeatedly accused Moscow of supporting pro-Russian separatists in eastern Ukraine and of attempting to destabilize other Ukrainian regions.
The SBU believes the detained Russians could be members of Russian intelligence agencies, SBU official Oleksandr Tkachuk told a briefing.
"Russian intelligence agencies are increasing their activities in peaceful (Ukrainian) cities," he said.
The United Nations said this week that arms and fighters were still pouring into rebel-held areas of eastern Ukraine from Russia and that the death toll from 20 months of fighting now exceeded 9,100.
The United States and European Union have slapped economic sanctions on Russia over its role in Ukraine.