The Ukrainian head of security services has been fired due to alleged "treason".
President Volodymyr Zelenskyy has also sacked prosecutor general Iryna Venediktova amid claims dozens of employees in her office have "worked against our state".
Mr Zelenskyy said: "Such an array of crimes against the foundations of the state's national security, and the links recorded between Ukrainian security forces and Russian special services, raise very serious questions about their respective leaders."
Ivan Bakanov, the head of the Security Service of Ukraine (known as the SBU), was a long-time friend of Mr Zelenskyy, according to Ukrainian news agencies.
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The news comes as former Russian president Dmitry Medvedev warned that any outside attack on the Crimea region would prompt a "Judgment Day".
Russia annexed the Black Sea peninsula of Crimea from Ukraine in 2014.
Moscow then also backed pro-Russian armed separatists in the Donbas region of eastern Ukraine.
According to the Russian TASS news agency, in the event of an attack on Crimea, Mr Medvedev said: "Judgment Day will come very fast and hard. It will be very difficult to hide."
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US warned against move that would 'endanger humanity'
Mr Medvedev's statement was issued a day after a Ukrainian official suggested Crimea, which most of the world still recognises as part of Ukraine, could be a target for US-made HIMARS missiles, recently deployed by Kyiv.
Medvedev has previously warned the US of the dangers of attempting to punish a nuclear power such as Russia over its actions in Ukraine, saying this could "endanger humanity".
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On the front lines on Sunday, Russian missiles continued to hit industrial facilities in the southern Ukrainian city of Mykolaiv, its mayor Oleksandr Senkevych said.
Mykolaiv has faced regular attacks in recent weeks as the Russians have sought to soften Ukrainian defences.
The Russian military has declared a goal to cut off Ukraine's entire Black Sea coast all the way to the Romanian border.
If successful, such an effort would deal a crushing blow to the Ukrainian economy and trade, and allow Moscow to secure a land bridge to Moldova's separatist region of Transnistria, which hosts a Russian military base.