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Kremlin spokesman Dmitry Peskov on Thursday called Nato "a machine for confrontation", advised against any expansion and lamented the "significant" loss of Russian lives in Ukraine.
He gave the most sombre assessment so far of its invasion, describing the "tragedy" of mounting troop losses and the economic effect of sanctions, as eastern cities in Ukraine were evacuated before an anticipated military offensive.
Russia had previously acknowledged its attack had not progressed as quickly as it wanted but Mr Peskov lamented the rising death toll.
"We have significant losses of troops," he said in an interview with Sky News. "It's a huge tragedy for us."
He was speaking minutes after the UN General Assembly voted to suspend Russia from the Human Rights Council.
Russia's invasion of Ukraine was in part to counter the expansion of Nato in Eastern Europe, yet in recent weeks the alliance has sent more troops to the region while Europe's largest power, Germany, has committed to increasing its defence spending.
Mr Peskov said Russia intends to "rebalance the situation" and "take additional measures" to ensure its security.
"It's not a peaceful alliance," he said. "It was tailored for confrontation and the main purposes of its existence is to confront our country."
At the outset of its invasion, Russian President Vladimir Putin spoke of "consequences greater than any ... in history" should the West intercede directly on behalf of Ukraine.
The threat was understood to be a statement of nuclear intent and Mr Peskov raised that again when talking of mutual deterrence should Finland and Sweden decide to join Nato.
"Should one side be more powerful than the other, especially in terms of nuclear arms, then it will constitute a threat for the whole architecture of security and it will [force] us to take additional measures to strengthen our credentials," he said.
"We will have to rebalance the situation. We'll have to make our western flank more and more sophisticated in terms of ensuring our security."
Asked if any Nato expansion would be seen as an existential threat by Russia, circumstances in which it has said it will use nuclear weapons, Mr Peskov initially appeared to assent before backtracking.
Mr Peskov repeated claims that Ukraine was using its civilians as human shields.
"They are covering themselves with civilian people and not letting them flee the town," he said.
He called satellite pictures of the massacre in Bucha fakes and part of a "conspiracy" and said Russia had provided "detailed explanations" on the matter.
"Of course, it can be a play of fakes. It can be a play of lies," Mr Peskov said. "We have to doubt sometimes."
He was less doubtful when asked if Mr Putin was worried about facing charges of war crimes in an international court.
"No, he's not," Mr Peskov said. "We don't see any possibility for that."
Speaking shortly after the interview, British MP Tom Tugendhat told Sky News that Mr Peskov's claims could not be believed.
"You know, the tragedy is that these lies aren't meant to be believed," said Mr Tugendhat, chairman of the House of Commons foreign affairs committee.
"What they're meant to do is they're meant to convince Russians that there is no such thing as the truth and that everybody's as bad as the other."
