Crimea or Donbas: Ukraine's chief spy on what will be harder to reclaim

"I believe that reclaiming Donbas will be more challenging because there is a continuous supply line, and their forces can access it from any direction. Those who once held Crimea loudly and clearly said that Crimea was an impregnable fortress, and everyone lost this fortress. Just as Ukraine did ten years ago," he said.

Budanov reminded that Russians still have a land corridor to Crimea today.

"The Crimean Bridge still stands, but its days are numbered. Two strategic tasks need to be solved: cut off the land corridor and destroy the Crimean Bridge. The rest is just a matter of time. The peninsula will be reclaimed. Situatin with Donbas is more complicated," he said.

According to the chief of the DIU, Russia will not dare to use nuclear weapons if it loses Crimea.

"Those who fear escalation say that if Russia understands that it will lose Crimea, it may start nuclear escalation. Do you expect me to scream in fear when I hear these words? There have been plenty of reasons for nuclear weapon use already, maybe dozens. And what do we have? In the Russian doctrine, nuclear weapons are seen as a 'means of deterrence.' This is what everyone fears and speculates about," Budanov said.

He emphasized that for Russia, the use of nuclear weapons will be much more costly than not using them.

Raids in Crimea

Last year, Ukrainian military carried out a series of successful operations in Crimea and its vicinity. There were also sorties to the occupied peninsula.

One of the most successful operations of Ukrainian intelligence was regaining control over the Boyko Towers - oil rigs near the shores of Crimea in the Black Sea, seized by Russia in 2015. Terrorists utilized the structure for military purposes since the beginning of the full-scale invasion.

For more details on how Ukrainian military managed such a special operation, please refer to the RBC-Ukraine coverage.