Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty

Kyiv's infrastructure could collapse at any second as Russia's sporadic missile attacks along with freezing winter temperatures put local authorities under increasing strain, the city's mayor Vitali Klitschko told Reuters.

"It's pretty cold in Ukraine right now so living without electricity and heating is almost impossible. The situation is critical. We are fighting to survive," he told Reuters.

Klitschko, who is attending World Economic Forum annual meeting in Davos, pleaded with allies to speed up deliveries of air defense systems.

"We don't talk about the collapse, but it can happen ... at any second (because) Russian rockets can destroy our critical infrastructure in Kyiv," he said.

The death toll from Saturday's missile attack on an apartment building in Dnipro has risen to 45, including six children, according to Borys Filatov, the city's mayor.

The search and rescue operations have ended, according to the emergency services.

In addition to the fatalities, there were 79 people wounded, including 16 children, according to emergency services.

Thirty-nine people were rescued from the rubble, including six children, emergency services said.

-ABC News' William Gretsky

Ukrainian soldiers arrived in the United States on Sunday to begin training on the Patriot air defense missile system at Fort Sill, Oklahoma, a U.S. military official said.

The training at Fort Sill is expected to last several months, and then switch briefly to Europe, officials said.

The U.S. military’s expanded combat training of Ukrainian forces in Germany began Sunday, Gen. Mark Milley, chairman of the Joint Chiefs of Staff, told The Associated Press.

-ABC News' Luis Martinez

Emergency crews have rescued 39 people, including six children, who were buried under the rubble caused by a missile strike on a high-rise apartment complex in Dnipro over the weekend, Ukrainian President Volodymyr Zelenskyy said in his Monday evening address.

The death toll remains at 40, including three children, he said.

The Kremlin denied being responsible for the attack, saying Russia doesn’t strike residential areas and claiming the destruction was a result of Ukrainian air defense.

"The debris of the house destroyed by the Russian missile is still being dismantled in Dnipro," Zelenskyy said. "I thank everyone who is carrying out this rescue operation. Every employee of the State Emergency Service and police, every doctor, every volunteer. Everyone who is involved."

-ABC News' Ellie Kaufman