(NewsNation) — Russia’s rush to mobilize hundreds of thousands of recruits to staunch stinging losses in Ukraine is a tacit acknowledgment that its “army is not able to fight,” Ukraine’s president said Sunday, as splits sharpened in Europe over whether to welcome or turn away Russians fleeing the call-up.

Speaking to U.S. broadcaster CBS, Ukrainian President Volodymyr Zelenskyy also said he’s bracing for more Russian strikes on Ukraine’s electrical infrastructure, as the Kremlin seeks to ramp up the pressure on Ukraine and its Western backers as the weather gets colder. Zelenskyy warned that this winter “will be very difficult.”

“They will shoot missiles, and they will target our electric grid. This is a challenge, but we are not afraid of that.” he said on “Face the Nation.”

He portrayed the Russian mobilization — its first such call-up since World War II — as a signal of weakness, not strength, saying: “They admitted that their army is not able to fight with Ukraine anymore.”

Zelenskyy’s comments on Sunday came amid more threats by Russian President Vladimir Putin to use nuclear weapons in the conflict. Zelenskyy said the international community should have little reason to doubt Putin will make good on those threats.

“I don’t think he’s bluffing,” Zelenskyy said. “I think the world is deterring it and containing this threat. We need to keep putting pressure on him and not allow him to continue.”

Putin earlier this week ordered a referendum in four large regions of Ukraine that are now occupied by Russian forces, asking citizens to choose whether to become part of Russia. If the regions are annexed, Putin has vowed to defend the areas by any means, including with the use nuclear weapons.

Virtually no nation would accept the eventual results as legitimate.

In the U.S., National Security Advisor Jake Sullivan said Russia is operating out of desperation.

“(Putin) does not believe that Ukraine should have a right to exist,” he said Sunday on NBC’s “Meet the Press.”

In the face of recent military setbacks on the battlefield, Putin ordered 300,000 more Russians be drafted into the war. The White House said it’s a sign of weakness but an indication the Kremlin has no intention of ending the war.

“The Russian army is in trouble, and the Ukrainian army is making gains on the battlefield and stopping the Russians from making progress in areas where they continue to try to advance,” Sullivan said. “But that doesn’t mean the danger is over, it is very much real and still with us.”

Sen. Dick Durbin, the No. 2 ranking Democrat in the Senate, told NewsNation even if Putin’s recent threats and actions prolong the war, Congress will continue to provide Ukraine with weapons, money and aid.  

“We expect to put more on the table in the years and months to come, I hope so,” Durbin said. “I think I’ll be part of a bipartisan group supporting that.”

The Associated Press contributed to this report.