NATO Secretary General Jens Stoltenberg has said that he believes Russia "has lost Ukraine forever" as a result of the war, and will not be able to achieve its war aims despite a massive military effort since the invasion in February last year.

Jens Stoltenberg, The North Atlantic Treaty Organization (NATO) Secretary General, answers questions at NATO headquarters during an interview with the German Press Agency

"The whole purpose of this invasion was to prevent Ukraine from moving towards NATO and the European Union. Ukraine is now closer to NATO and the European Union than ever before," he told dpa in an interview shortly before Christmas. "This is a big strategic defeat for Russia."

"President Putin has lost Ukraine forever," said Stoltenberg, referring to the fact that Russia saw Ukraine as part of its sphere of influence for decades.

The former Norwegian prime minister is certain that Ukraine will eventually achieve its goal of joining NATO.

He also pointed out that Russia was paying an enormously high price for its war.

"Russia has lost hundreds of planes, thousands of battle tanks, and 300,000 casualties. Their economy is weaker. They are politically more isolated," he said.

"Inflation is up, living standards are down," he said.

However, Stoltenberg warned against counting on a quick end to the war, saying that he had no indication that Putin would change course, even after his expected re-election on March 17.

Ukraine meanwhile has "demonstrated they're able to defend themselves, they're able to fight back as long as, especially when they get weapons from Germany and many other NATO allies."

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A former CIA analyst says Putin could launch a nuclear weapon against NATO territory.

• An ex-CIA analyst says Russia might risk nuclear war with NATO to prevail in their war with Ukraine.

• Peter Schroeder says Putin has been "remarkably consistent" about using nuclear weapons on NATO.

• Putin said recently that Russia had no interest in fighting NATO forces.

An ex-CIA analyst says the US might be underestimating the possibility of nuclear war between Russia and NATO.

"That is a mistake. US officials have it backward," Peter Schroeder wrote in an article for Foreign Affairs published on Wednesday.

"It is actually quite unlikely that Russian President Vladimir Putin will use a nuclear weapon on the battlefield in Ukraine, but it is very possible that he will move toward using one against NATO," Schroeder added.

The former public servant spent nearly two decades working on issues relating to Russia. Besides working for the CIA, Schroeder had done stints at the US embassy in Moscow and the National Intelligence Council.

"When Putin invokes his arsenal, he is not trying to warn that Russia could use tactical nuclear weapons in Ukraine. Rather, his rhetoric is designed to threaten NATO itself," Schroeder said in his article.

Schroeder said that Putin wasn't just engaging in saber-rattling whenever he touted Russia's nuclear capabilities. He believes that going nuclear is something Putin could actually do if he wanted to win the war in Ukraine swiftly.

"Unlike the West, Putin may not fear a nuclear standoff: he is well-versed in Russia's nuclear arsenal and the tenets of nuclear deterrence, and possibly sees himself as uniquely suited to navigating a nuclear crisis," Schroeder wrote.

"And Putin has been remarkably consistent that Russia is willing to use nuclear weapons against NATO to defend its interests in Ukraine," he added, referencing Putin's remarks eight years ago when he said Russia was ready to mobilize its nuclear forces during its invasion of Crimea.

Schroeder then outlined how Putin could ratchet up Russia's pressure against NATO.

"If Putin does escalate the war, for instance by attacking NATO with conventional weapons, he will likely move very swiftly, so as not to give the United States a chance to maneuver away from a crisis," he wrote.

"From there, Moscow might actually begin using force against NATO," Schroeder said, adding that Russia could attack NATO aircraft, ships, and convoys to drag NATO into the fight.

"Finally, in a worst-case scenario—one where the Kremlin sought to shock the world into ending the war in Ukraine quickly and on Putin's terms—Russia could actually launch a nuclear weapon directly at NATO territory," Schroeder continued.

Both the US and Russia have sought to downplay the possibility of nuclear war.

In July, US President Joe Biden said at a press conference in Helsinki that there wasn't any danger of Russia using nuclear weapons on Ukraine.

"I don't think there's any real prospect, you never know, of Putin using nuclear weapons," Biden said then.

Last week, Russian leader Vladimir Putin brushed aside Biden's comments on how Russia would attack NATO after defeating Ukraine.

"Russia has no reason, no interest, no geopolitical interest, neither economic, political nor military, to fight with NATO countries," Putin said in a televised interview on Sunday.

Representatives for Schroeder, the White House, and the Russian foreign and defense ministries did not immediately respond to requests for comment from Business Insider sent outside regular business hours.