Ukraine's state security service (SBU) said they arrested a Russian woman accused of being a saboteur sent by the Kremlin to stir up unrest and violence across the border.
Maria Koleda, 22, was detained in the southern Ukrainian city of Kherson, from where she was overseeing the setup of armed subversive groups and spying on Ukraine's defence forces deployed at the border with Crimea, the SBU said.
Koleda was also allegedly involved in this week's clashes in the city of Mykolaiv, during which pro-Russian protesters stormed the regional administration offices.
The SBU said the woman, who visited Ukraine nine times in recent months, admitted to firing a weapon, injuring three people.
Earlier this week, Mykolaiv rioters were pushed back by supporters of the revolutionary government in Kiev and security forces, which also dismantled their protest camp.
Announcing Koleda's arrest, the SBU also linked to her purported personal account on VKontakte - the Russian equivalent of Facebook - on which she made no secret of her pro-Moscow activities in Ukraine.
In her last post, dated April, 8 she writes: "People of Mykolaiv! Don't despair! Yes, today you were taken out, the Maidan idiots crushed the tented camps, many wounded and injured ... Do not worry! Everything is ahead, because life goes on! #Mykolaiv #Ukraine #antimaidan."
In recent days, Koleda, who is seen wearing paramilitary gear while holding a rifle in her profile picture, posted tens of photos depicting barricades, protest camps and demonstrations that she said she took in Kiev and other Ukrainian cities.
Hers was the second arrest of a Russian citizen accused of artificially fomenting chaos and separatism in eastern Ukraine announced by the SBU in April.
Kiev and the West claimed that Russian agents have been the catalyst behind a recent wave of pro-Moscow unrest in eastern Ukraine, where many ethnic Russians live.
US Secretary John Kerry said that Russia was trying to destabilise its neighbour to justify a possible invasion.
"This could potentially be a contrived pretext for military intervention just as we saw in Crimea," Kerry said.
Russian troops moved into Crimea shortly after the pro-Kremlin Ukrainian president Viktor Yanukovich was toppled by a revolution in February.