Ukraine's President Volodymyr Zelenskiy sacked the head of the country's domestic security service and state prosecutor, citing hundreds of cases of alleged treason and collaboration with Russia, as Moscow appeared set to step up military operations.

Zelenskiy said more than 60 officials from the SBU security service and prosecutor's office were working against Ukraine in Russian-occupied territories, and 651 treason and collaboration cases had been opened against law enforcement officials.

The sackings on Sunday of Ivan Bakanov, head of the security service, and Prosecutor General Iryna Venediktova, who led efforts to prosecute Russian war crimes, and the sheer number of treason cases reveals the huge challenge of Russian infiltration as Kyiv battles Moscow in what it says is a fight for survival.

"Such an array of crimes against the foundations of the national security of the state ... pose very serious questions to the relevant leaders," Zelenskiy said. "Each of these questions will receive a proper answer.

In his nightly speech to the nation, Zelenskiy noted the recent arrest on suspicion of treason of the SBU's former head overseeing the region of Crimea, the peninsula annexed by Russia in 2014 that Kyiv and the West still view as Ukrainian land.

Zelenskiy said he had fired the top security official at the start of the invasion, a decision he said had now been shown to be justified.

"Sufficient evidence has been collected to report this person on suspicion of treason. All his criminal activities are documented," he said.

Reuters