President Volodymyr Zelenskyy of Ukraine has sacked the head of the country's security service (the SBU) and the country's prosecutor general, citing hundreds of criminal investigations into treason and collusion involving employees of their departments.

"Today, I made the decision of relieving of their duties the prosecutor general and the head of Ukraine's security service," Zelenskyy said in his daily address to the country on Sunday.

Over 650 cases of suspected treason and aiding and abetting Russia by Ukrainian security officials are being examined.

"More than sixty employees of the prosecutor's office and the SBU have remained in the occupied territory and continue to work against our state," Zelenskyy stated.

"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 grave questions about their respective leaders," he said.

Dismissed Oleksiy Symonenko has replaced Iryna Venediktova as prosecutor general of Ukraine.

According to Ukrainian news agencies, dismissed SBU director Ivan Bakanov was a longtime friend of Zelenskyy.

Moscow warns of impending doom

Meanwhile, Dmitry Medvedev, the deputy head of Russia's Security Council, which President Vladimir Putin chairs, responded to statements by Ukrainian officials that Kyiv may strike the bridge connecting Crimea and Russia by stating that such an action would have devastating consequences for the Ukrainian leadership.

"If that happens, the consequences will be obvious: They will momentarily face Doomsday," stated Medvedev on Sunday. It would be quite difficult for them to hide.

Medvedev, formerly considered more liberal than Putin by the West, stated that Russia would continue its actions in Ukraine until achieving its stated objective of "denazifying" and "demilitarising" the country.

He predicted that the ongoing violence in Kyiv would "undoubtedly lead to the collapse of the existing regime."

Kyiv has accused Moscow of initiating new attacks on eastern and southern Ukraine residential neighborhoods.

Moscow announced it would increase its military actions, and Kyiv accused Russia of deploying missile launchers at Europe's largest nuclear power plant, prompting the latest attacks.