In a dramatic escalation of East-West tensions over Russia's invasion of Ukraine, President Vladimir Putin ordered Russian nuclear forces put on high alert Sunday in response to what he called "aggressive statements" by leading NATO powers.

The order means Putin has ordered Russia's nuclear weapons prepared for increased readiness to launch, raising the threat that the tensions could boil over into nuclear warfare. In giving it, the Russian leader also cited hard-hitting financial sanctions imposed by the West against Russia, including Putin himself.

Speaking at a meeting with his top officials, Putin directed the Russian defense minister and the chief of the military's General Staff to put the nuclear deterrent forces in a "special regime of combat duty."

"Western countries aren't only taking unfriendly actions against our country in the economic sphere, but top officials from leading NATO members made aggressive statements regarding our country," Putin said in televised comments.

The Russian leader this week threatened to retaliate harshly against any nations that intervened directly in the conflict in Ukraine, and he specifically raised the specter of his country's status as a nuclear power.

The U.S. ambassador to the United Nations responded to the news from Moscow while appearing on a Sunday news program.

"President Putin is continuing to escalate this war in a manner that is totally unacceptable," Ambassador Linda Thomas-Greenfield said. "And we have to continue to condemn his actions in the most strong, strongest possible way."

The alarming step came as street fighting broke out in Ukraine's second-largest city and Russian troops squeezed strategic ports in the country's south, advances that appeared to mark a new phase of Russia's invasion following a wave of attacks on airfields and fuel facilities elsewhere in the country.

The capital, Kyiv, was eerily quiet after huge explosions lit up the morning sky and authorities reported blasts at one of the airports. Only an occasional car appeared on a deserted main boulevard as a strict 39-hour curfew kept people off the streets. Terrified residents instead hunkered down in homes, underground garages and subway stations in anticipation of a full-scale Russian assault.

"The past night was tough - more shelling, more bombing of residential areas and civilian infrastructure," Ukrainian President Volodymyr Zelenskyy said. "There is not a single facility in the country that the occupiers wouldn't consider as admissible targets."

Following its gains to the east in the city of Kharkiv and multiple ports, Russia sent a delegation to Belarus for peace talks with Ukraine, according to the Kremlin. Zelenskyy suggested other locations, saying his country was unwilling to meet in Belarus because it served as a staging ground for the invasion.

Ukrainian forces secured full control of Kharkiv on Sunday following street fighting with Russian troops in the country's second biggest city, the local governor said.

"Kharkiv is fully under our control," the head of the regional administration, Oleg Sinegubov, said on messaging app Telegram, adding that the army was expelling Russian forces during a "clean-up" operation.

Earlier Sunday he had said that Russian forces' light vehicles broke into the city, with fighting breaking out in the streets. Sinegubov said that Russian troops were "absolutely demoralized."

He said Russian forces were abandoning their vehicles "in the middle of the road" and groups of five were surrendering to the Ukrainian army.

"As soon as they see at least one representative of the armed forces, they surrender," Sinegubov said.

He said that "dozens" of Russian soldiers have already surrendered.

"The captured Russian fighters speak of complete exhaustion and demoralization, they have no connection with the central command, they do not understand and do not know what they do next," the Kharkiv governor said.

"Since the beginning of the attack on Ukraine, they have not received food and water," he added. "Leaving the positions, Russian fighters are trying to hide among the civilian population, asking people for clothes and food."