Russia has selected and prepared a puppet government to install in Kyiv once its forces seize the Ukrainian capital, a senior U.S. administration official told ABC News.

U.S. intelligence believes Russian troops will ultimately crush Kyiv and decapitate its government, amid mounting evidence of indiscriminate shelling and a barrage against civilian targets across Ukraine, according to the official.

The official expressed concern that Ukraine lacks air power and what air force they had has been attacked, allowing Russia to mass its forces en route to Kyiv.

-ABC News' Martha Raddatz

Russian forces are advancing on Mykolaiv, another key city in southern Ukraine, the regional governor warned Friday.

In a video message posted on social media, Mykolaiv Oblast Gov. Vitaliy Kim said Russian troops are moving on Mykolaiv city from two directions and that some have already entered the city limits but are not yet inside in significant numbers.

The city is preparing to defend itself, according to Kim.

"Don't panic," Kim said. "At the moment, the enemy is approaching from two directions but they're not on our streets yet. We're preparing the defenses, so women and children should get home now and the men join the defense lines."

Kim said the Ukrainian military has a large amount of armour in Mykolaiv and urged residents "not to shoot at every vehicle," since some could be Ukrainian.

"No need to shoot at everything that's moving in the city. There's a lot of our armor in the city," he said. "{lease do not shoot inside the city, there's no enemy here yet, but they are approaching."

-ABC News' Patrick Reevell

The U.S. Embassy in Kyiv took to Twitter on Friday to condemn Russia's shelling of Ukraine's Zaporizhzhia nuclear power plant, the largest in Europe.

"It is a war crime to attack a nuclear power plant," the embassy tweeted. "Putin's shelling of Europe's largest nuclear plant takes his reign of terror one step further."

NATO Secretary-General Jens Stoltenberg met with U.S. Secretary of State Antony Blinken and other NATO foreign ministers in Brussels on Friday morning to discuss the response to the ongoing Russian invasion of Ukraine.

During a press conference prior to the meeting at NATO headquarters, Blinken and Stoltenberg condemned Russia's attacks on civilians in Ukraine and expressed concern over the reports of Russian shelling at Ukraine's largest nuclear power plant.

"This just demonstrates the recklessness of this war and the importance of ending it, and the importance of Russia withdrawing all its troops and engage in good faith in diplomatic efforts," Stoltenberg told reporters. "We provide support to Ukraine. At the same time, NATO is not part of the conflict. NATO is a defensive alliance, we don't seek war conflict with Russia."

Blinken emphasized that NATO and the United States "seek no conflict."

"But if conflict comes to us, we're ready for it," he added. "And we will defend every inch of NATO territory."