A Russian expert told ABC News Saturday that recent threats by Russian President Vladimir Putin to use nuclear weapons in its war against Ukraine is putting the world in "uncharted waters."

"We're in uncharted waters," John Hardie, deputy director of the Foundation for Defense of Democracies' Russia Program told ABC News Saturday. "I think this would be very much a last resort, but I think we're a long way from that point."

Hardie was responding to Putin’s mobilization of 300,000 troops to combat recent Ukrainian military gains on the ground in Ukraine.

In a televised address announcing the call-up, Putin said that Russia would use all its destructive power to "defend" its territory from any Ukrainian or Western incursion.

"In the event of a threat to the territorial integrity of our country and to defend Russia and our people, we will certainly use all means available to us," The Associated Press reported Putin saying in the address Tuesday. "This is not a bluff."

Former State Department official Max Bergmann, told ABC News that he feels the use of nuclear weapons "is something you have to take very seriously."

"Russia has the world's largest nuclear arsenal," said Bergmann. "And when the Russian president starts making nuclear threats, it's something everyone has to pay attention to."

Bergmann, director for the Europe Program at the Center for Strategic and International Studies, said that Putin is risking his regime by drafting men to fight in the war, which he is still calling a "special military operation."

"What is clearly happening here is that the Russian military position in Ukraine is collapsing. Forcing people to go and fight in Ukraine is an extremely risky political decision. This is one of the most incredibly disruptive things that can be done to a society," Bergmann told ABC News Saturday.

Another expert told The Associated Press that the combination of Putin holding referendums on annexing parts of Ukraine sympathetic to Russia, his mobilization, and nuclear threats, are part of a larger strategy to keep the West at bay.

"This is a blunt Russian ultimatum to Ukraine and the West: Ukraine must back off or there will be a nuclear war," Tatiana Stanovaya, an independent political expert who follows the Kremlin’s decision-making told the AP.

"For Putin, the annexation would legitimize the right to resort to nuclear threats to protect the Russian territory."