The Security Service of Ukraine (SBU) says one of its top officials has been detained on suspicion of collaborating with Russia's Federal Security Service (FSB).
In an April 14 statement, the SBU said Major General Valeriy Shaytanov is suspected of high treason and terrorism.
It said audio and video recordings gathered during the case provided "irrefutable evidence" of the crime.
According to the SBU, Shaytanov started working for the FSB in 2014 and had planned "terrorist acts" in Ukraine, including allegedly planning to murder Adam Osmayev, the leader of Chechen volunteers fighting on the Ukrainian side against Russia-backed separatists in Ukraine's east.
Osmayev was injured and his wife was killed in an ambush near Kyiv in October 2017 that Ukraine blamed on Russian agents.
Shaytanov also allegedly provided information to Moscow about secret operations against Russia-backed separatists in eastern Ukraine and recruited additional agents, the SBU said.
Ukrainian-Russian relations have been strained since 2014 when Russia seized Ukraine's Crimea region after sending in troops and staging a referendum deemed illegitimate by at least 100 countries.
In April that year, Russia threw its support behind armed separatists in Ukraine’s eastern regions of Donetsk and Luhansk, where some 13,200 people have been killed in the ongoing conflict.